What are you thinking?????
Why do you have to be responsible to this boy who is turning 18? Is it because he got your daughter pregnant? Is your daughter asking you to let him stay so they "play house"? And why is he being kicked out of his home?
Your responsibility is to YOUR family, not anyone else's.
You ask what the house rules should be. There shouldn't be any house rules for him because HE SHOULD NOT BE MOVING IN! His presence is bound to cause a disruption in your family. And it might even break the bond between you and your daughter. She, most likely, will start siding with him when it comes to certain things. Arguments and tension are soon to follow. Will he leave if things get bad? Or is it possible that he will throw the "I'm not leaving without my child" in your face.
Before you truly agree to this and ALLOW it to happen, really think of ALL the possiblities that can arise and not just the "live as one big happy family" one.
Seriously reconsider your offer.
M.