Wow! Huge major changes all at once. Not only a move, change of jobs, but moving in together with secondary and his girlfriend. Breath!
Have you guys talked about your living arrangements? Division of houshold chores, space, finances, etc? What happens if things blow up and someone decides to leave?
You mentioned that they pretty well suck at communication. It's not your job to micro manage their relationships, but you might want to make sure everyone is having "family meetings" regularly (weekly) to discuss stuff that effects the entire household. Everyone is going to have to learn to communicate with an entire new group of personalities, it wouldn't be a bad idea to discuss how each of you communicate from the start.