Probably not. I think you are lucky that he seems aware that there can be problems and is making efforts for you to get a trial run. Most fathers just assume that women will fill that role AND jolly well enjoy it too!

You seem to have a lot of reservations and doubt already. I think you should listen to that little voice inside.

Does BM have any rights? Is co-parenting going to be the 3 of you? If it is just you and your SO as the parents, you have a MUCH better shot at making this work. It get's much more difficult when there are different households involved. I have a great relationship with my kids SM and BD. We work through most things together when it comes to our kids. However, there are times that even in a peaceful coparent relationship where you want to wring their necks. 

Before you get married, you should see if you can handle this. If you are having this many doubts now and don't want to move in-- you probably shouldn't get married. 

He's being smart tbh. That gives you the chance to experience it before tying the knot.... It makes sure in this situation that everyone is a smart fit for each other.

So look at what you want. Because no, being positive doesn't make stepparenting smoother. In fact, when I was bieng overly positive and happy go-lucky, I think that's what hurt the worse... You have to be realistic about it.

Keep your own place during this "trial." If you don't, you could end up feeling stuck at his place.

I would do some soul-searching and see if you really want to be in the mother role. With full custody, it's harder to disengage if things become too hard. Also, if you do decide to move in and do the trial thing, make sure you have serious discussions about what his expectation of your role is, and what you are and aren't willing to do. Does he really want you to be their mom? Do you want that? Does he expect you to cook, clean, etc? Will you be allowed to discipline skids, and will he discipline them? Will you have input on rules in the house and skids contributing (chores, picking up after themselves, etc)? Can he still make time for just the two of you to have a relationship where skids aren't involved? Make sure you are on the same page (or can get on the same page) with how you want to parent skids before you accept this responsibility.

Those are the kinds of things that cause resentment and big issues in households where one person is not the Bio parent. It is really hard being a step-parent, but even harder if you have no say and the dad feels guilty so lets skids do what they want with no discipline.

However hard you think being a SM is- multiply that by at least 2. I never thought it would be this hard. Things come up that you never imagined. I love my SO, but would not choose someone with kids again if I ever end up single.