You had 4 yrs to discuss all things needed to blend your two families. If she arrives at the border with a different address then you and they ask about why you all will be living seperate. You can bet your last dollar that she will be on the next flight out. If they catch any whiff of something off they will turn your fiance right back.
The visa as mentioned is to get married and adjust asap. They give you 90 days because sometimes it can be difficult to get a JP or someplace to get married. It is not a try before you buy visa.
Kids will do better adjusting when they aren't living in a temporary place and then put into the next place and wondering what is next. I moved here with 4 kids ages 8-16 I can tell you for sure if we lived in a temp home first then moved to my then fiance's home it would still be just as much of a learning curve for all involved. It takes months, possibly years to adjust. 90 days will not cut it.
Have you talked about expectations? Discipline? Bedtimes? From experience you discipline your kids and only your kids, do not step on the other parent's toes. The other parent handles their own kids. THis does not mean each other kids can be disrespectful at all. YOu two really need to come up with parenting plans. This can be easy or hard depending how you both handle things and the kids involved. If any have any type of learning, special needs, behaviour, disabilities and the other parent is not used to it that alone can cause issues. The way one handles a child who is not like the others can vary and many times if the other person has zero experience with things like that they will not understand.
Check this forum too we have many posts like this about kids.