I've got a pretty small build as well, but I hold up just fine in fights. You have to fight with your brain, not your brawn, when you're dealing with physically superior opponents. Be quick, dont let them hit you - tai chi and Aikido classes can help you learn how to use the other guy's momentum and strength against him to throw him off balance. I've literally seen a 90 pound, 50 year old lady throw a 200 pound, 30 year old guy across a room - it's all about balance and momentum.
That's assuming you ever actually have to get into a physical conflict, though. If you want to avoid that altogether, go talk to the Gym owner, tell him to watch the camera recordings, and to fire his night guard for harassing customers. Have him send those recordings to the police as well, and you can get a restraining order against all of the guys involved. If you're worried about anything happening away from the cameras, buy yourself a knife, tazer, or concealable gun, and keep a micro camera on you so that you have evidence that any incident in which you had to defend yourself was exactly that - self defense.
