When your're ready to do the first overniter are you close? If they're not in the bedroom could you sleep close to them? Even if it means a sleeping bag on the floor?
That would kind of make everything safe and ease your mind too.
Good thing with ratties is that they are usually going to sort things out at night being nocturnal so if the night goes ok you can have hopes for the day, right? But no guarantees... better have a vet fund saved if you intend to do intros.... :?
Sorry, not being negative, just being inexperienced. and scared raisin-less.