Khell, that idea is awesome and all, but the whole thing about all the people wearing paintball masks would just break it. I would give everyone some protective glasses, but not a full-blown mask. You would have to have a disclaimer stating that there will be fake gunfights, and that people might take minor injuries. That's what I would do. I would also have a rule against hitting people in the face. Since the game is already expensive, maybe have a fine fine for the first two offenses, and disqualification on the third.
Also, there might be a problem with the security threat that it presents. If you've got fifty people being fake terrorists, then one person who happens to be a real terrorist would have an extremely easy time getting in. This could be mostly fixed with metal detectors placed sporadically about, and having a brief search of people entering the mall. Nothing big like airports, but still, security.
Another problem would be (I don't know about the mall you speak of) that most malls have multiple floors. Intense situations like this would make it easier to fall down stairs or escalators, or fall ever rails and such. This could be fixed with a net or something for the rails. The stairs, though, you could have a rule against shooting in or shooting into certain areas where it would be dangerous to do so.
Other than these small flaws, the game sounds fun as hell.
