As someone with an abusive ex. The best way to get along for the benefit of the children is to learn what's called parallel parenting.

Children adjust very easily to having two different households with different rules, especially when they are that young.

Parallel parenting will minimize conflict between the parents while maintaining healthy boundaries.

Do NOT get involved in co-parenting, it's not your responsibility.  She will have to set clear boundaries and get legal protection as needed.  They had a child together, so he will likely be around and she will need to figure out how to manage that. You can support her, but don't take over, it won't work.

Also - you don't know her well, be sure you've seen evidence of him being abusive and aren't just believing her.  Many of us are with loving, caring men whose exes have accused them of being abusive, when the reverse was true - it was the woman who was abusive.  So keep your eyes open for red flags about BOTH of them.  Don't let love blind you.  Many abusive women go into new relationships playing the victim and claiming their exes are abusive.  So have you seen him act abusive towards her? Have you seen nasty, abusive emails or texts from him? If not - take it all with a grain of salt. I know that's hard to do when you love someone, but it really is important.

Also - be careful with your birth control.

Do you really want to be bogged down by a child that isn't yours and a potentially abusive situation?

Do you have a n education/training of some sort?  A career of your own?  That would be a better and more rewarding way to spend your time than getting involved in this situation, which already has red flags.

How fresh is "very fresh"?  I don't think it's a new boyfriend's place to be co-parenting with their step kids other parent.  Leave the parenting to the bios.  

The first thing you need to do is move VERY slowly with this relationship.  As others have pointed out, you likely don't know the whole story surrounding her relationship with her EX.. and many other things about her.  Date her.. fine.. but don't rush to fill the "daddy spot" in her child's life.. the first order of business is to scratch below the surface picture you are being shown and get to know her.. That also means lookiing for red flags and being ready to cut and run if necessary.

You are hearing her side of the story.. unless you witnessed or have seen court records confirming.. the version of the truth you are getting is hers.. you don't know his.. and you don't know where the absolute truth lies.  I am not saying that you go into this thinking she is a liar.. of course, she deserves you having an open mind.. but to an extent.. it will be helpful if you give that same open mind towards her EX.. a man that will be in your lives for quite a while if you choose to stay with her.

But.. that is the other issue... coparenting... To be blunt.. you are not a parent.  You are not this child's parent... The child has two living parents and THEY are the ones that are responsible for parenting the child.  Do not dive in to play daddy.  don't get involved in conference calls with her EX over issues surrounding the child.  Your only role is to be a support to your partner.  That might involve helping her do things related to the child.. like picking up from daycare or something.. but the child is their absolute responsibility.. including FINANCIAL.  Anything you shell out for that child is a gift and should be 100% voluntary.

Please do not try to swoop in and "audition for the daddy role" for this child.  A positive and friendly interraction is fine.. but how confusing will it be if it doesn't work out and you leave.. harder on everyone then.  Better to hold back and be a more fun uncle type of relationship than a parenting one for you.. at least right now.. if you are together for years and eventually marry.. perhaps your role in the kid's life will be more parental.. but now.. don't go that direction.

So.. for now...

1.  Don't move in quickly with them

2.  Don't get her pregnant 

3. Don't overstep into the father role for this child

4.  Let the parents parent.   Your only role is to support your partner... you can help with tasks.. or a sympathetic ear.. but you do not fight her battles for her.  

5.  Be respectful to her EX when you meet.  You don't have to be best friends with him.. but you don't need to go in there creating an issue acting like her big, bad bodyguard.  Trust her to deal with her ex as she safely sees fit.  If he harms her.. encourage her to seek legal assistance.

6.  Get to know her.. look for red flags that will bring drama into your life for years.. you are very young.. you have lots of options that don't come with a child and an EX.  Carefully consider that.  

As in what does it actually mean where the ex is completely in the picture?  Having a four person relationship?

What is your feelings on this ? 

A few months more than 27 years ago I met my DW and my SS-28, then 15mos old. So, my situation was much the same as yours is now.  While there was never any physical abuse of my DW by the SpermIdiot there was fairly comprehensive emotional abuse by the SpermClan nearly as a whole.  The SpermIdiot and the SpermGrandHag were the primary source of the gaslighting, attempts to isolate DW and SS from her family, and a constant level of pressure and blame on DW for not being a good enough "wife" (they were never married) to keep the SpermIdiot from cheating his ass off with ever younger underage statutory rape victims while my future DW was pregnant with SS and then during SS's first year of life.  

I entered the picture when I was 29.  So had quite a few years of road miles that you have yet to experience.  My DW was 18.  We married a week before SS-28 turned 2yo.

I did not take over the coparenting with the SpermIdiot/SpermClan.  There really was no coparenting.  Though it was not a conscious decision what we lived for the duration of the 16+ years under a CO was parallel parenting.  My DW was awarded full physical and legal custody at birth because the SpermIdiot was not named on the birth certificate.  A year later my ILs pushed my DW to have paternity established officially so they engaged an attorney who filed for paternity and CS.  My DW had SS when she was 16.  She graduated from HS a year later and a few months later moved out of State for university.  She was in a reconciliation loop with the SpermIdiot who never did what he said he would do so... DW ended any reconciliation discussions and only talked with the SpermIdiot about coming to visit his son.  That never happened. Not once in SS's entire life.  

Once we married I had her back. That was my role. She and I are equity life partners which made us equity parents to any children in our marriage regardless of kid biology. As it turned out, SS-28 is an only child in our marriage.  My DW handled all interface with the SpermClan... until I felt it was necessary to deploy the big stick to get them back under their slime covered rock at the bottom of their shallow and polluted gene pool.  They did not like it when I felt the need to get involved. My DW rarely needed my help, but when she did, I was there.

Parallel parenting is the way to go IMHO. That leaves only visitation hand offs to deal with and dealing with any required changes as events unfold over the next 16 years.  Don't over complicate things.  Our situation was kept very stable and fairly simple because we never lived nearer than 1200 miles from SpermLand or anyone in the SpermClan.  Visitation was on the long distance model and was 5Wks summer, 1Wk winter, 1Wk spring.  My DW had all holidays except for July 4th which could be the Sperm Clan's depending on when they chose to initiate their summer 5wk visitation.  The CO gave my DW every Christmas day so winter visitation alternated between the day school was out and Dec 24 or Dec 26th until the day before school started alternating between even and odd years.

My DW and I were completely on board as equity life partners and as equity parents.  We also were in complete agreement that the blended family opposition had zero place or say in our family.  Our partnership meant that we were dedicated to the well being of the kid including minimizing the negative influence of his SpermClan and total confrontation of any of their manipulative lying bullshit.  Though for the first several years of our marriage my DW was nonconfrontational on the naive perspective that if she gave them what they wanted that they would not take any crap out on the toddler Skid.  Wrong.  I finally was able to point out that the more she gave the, the more manipulative, toxic, and nasty they were with her and with the Skid. So... she planted her flag on that hill to die on and committed to keeping them in their place.  At that point their influence over our marriage and in SS's life was strictly limited and controlled.  We never denied visitation and in fact when they cried poor mouth about not being able to afford visitation travel we would pay their part of visitation travel expenses when it had been a year of more since SS's had been on SpermLand visitation.   

When they did start their manipulation of SS we would counter that with the facts and as he got older we reviewed all of the documented history and facts so he would have the information necessary to make his own choices and develop an understanding of reality rather than the bullshit the SpermClan fed him.  

With the BIoDad in the opposition in your blended family situation I would recommend a similar strategy.  Have your SO's back. If her X pulls any bullshit, make it clear that you will not tolerate any manipulation of your mate, your Skid, or the family that you have with them.  DW should be the primary point of interface, but.... as necessary you can show the big stick and make it clear to daddy that his crap, if he tries it, will be met with as much legal, financial, and social force as is necessary to keep him under his alleged characterless abuser rock.

I do agree that you should be very observant of the overall situation including your own SO and her X.  Most likely, reality lies somewhere in the middle between him being a full blown abuser and just an asshole.

As this relationship unfolds, you will find where reality lies.

Yep, even I have some areas where I can see and recognize the possibility of gray.

However, based on your initial post, I will go with the situation being as you describe.

Take care of you.  Enjoy making an amazing life and family with your SO and the Skid. It can happen.

Good luck.