In all honesty, if they get a cat without your and/or your other roommate's approval, tell the manager. Or threaten them by saying that you will tell the manager. (hey, I know it sounds childish, but at least your covering your own...) If they do not respect not only your and your other roommate's wishes, plus the "rules" by not having a cat, then actions need to be taken.
And if all else fails, move to Cali! I've got a few couches... hehe
Good luck!
LITP~Beth