Ideally, you do not have children with a person who has a different set of expectations as a parent.

Even if SS didn't exist... would you want to be the "bad guy" with your child enforcing all these rules that "daddy" doesn't care about?

This is central to the concept that your partner is the problem here.  It's not that the child doesn't keep a neat room.. he has never been taught.. he has never been told this is important.. it has never been expected of him.  It's not his fault period... this is not a character flaw of the child.. but of both his parents that allow it.

Why do you want to have kids with a person who has shown they are not an effective parent.. a lazy parent if you will?

I'm going to edit to say.. I went back and re-read another blog.. and you have already had a child with this guy.  so.. honestly.. it's not so simple as to have different rules for different kids... because it will be your DH that would have different rules for his own kids.. it's easy to say.. "your kid your rules".. but your child is also his.. if he truly doesn't care about room status.. I'm not sure how you are going to expect him to back you up on this with your child.  He may just not think that a messy room is the end of the world.. and may put his priorities elsewhere.. raising a nice kind child.. vs one worried about being in perfect order.  

This is a convo you will have to have with him.. to ask how he expects to handle it...

Your daughter is much younger than SS, 8 years or so if I read your posts right. By the time she's old enough to think about his room being messy, he will probably not be visiting anymore, given that he hates it at your house. And if she does notice, then you say, "Because he has a different mother."

From my POV there are going to be different rules with different parents. Just set some rules with your bio. That is the best you can do.

I find it's not that big of an issue. As DS gets older he actually understands why. He is just as repulsed by SKs behavior as I am. 

Also I do things for BS I would never do for SKs for the same reason they have a different mother. It goes both ways SKs may think they have the upper hand by not having rules until they are excluded from and activity because I'm not thier mother I want to to have an enjoyable time and don't want to deal with feral children.

I agree it goes both ways. DH and I agree on some central parenting issues, but ultimately I disengage from his and we each parent our own. Mine as well notice stepkids' bad behaviors and know I hold mine more accountable. They have never complained to me for being stricter on them, if anything have been horrified by the stepkids behavior. They get it.
 

I also reward mine for good behavior by taking them out for activities. I'm too embarrassed to take out my stepkids in public anyway and would not enjoy the time. 
 

My kids do have some different rules than his and mine are also here full time. DH are 50/50. During the pandemic I have allowed mine to have their own TV's in their rooms. We have a big family and they need their own space. DH didn't want his to have one in their room so that's fine. You could say maybe we should have agreed on this to be unified.

But firstly mine are here all the time and his go back and forth. His have their own TV's at TM. Secondly, mine are responsible and can handle it. I know his complain about things they don't think are fair. They are poor innocent victims in their eyes. But I don't care. If I were on my own I would do what I want for my kids and I'm not going to change that because of the stepkids. Might sound selfish but I've had so many issues with them I now refuse to change my life too much around because of them.