As far as I can tell, roommates will use any facility in the house unless you lock them out of the particular area. I would recommend making their personal bedrooms with all their personal items in the room and lock the door.
I have tried this with multiple files and have noticed a glitch in my game. I invite a Sim in as a roommate, saved the game, and shut it down. The next time I turn on the game, they have turned into an active member of my household rather than a uncontrollable roommate. Has anyone else had this problem? I don't really want to have to get new roommates every time I start a file.