My campaign features the PC's mostly working for a powerful crime family. As such, they end up doing some rather bad things...all the time. I hadn't really considered the possibilty that it would be a problem, but i was thinking it might be one if someone wants to be a righteous paladin or something. You guys think this may have to be changed for that possibility? I'd rather not tell them what kind of character they can make.
Well, the first thing is that while your sentiment to not tell the players what kind of character they can make is admirable, in practice a DM needs to excercise judgment over allowing characters into the campaign and needs to properly heads up the players about what sort of characters they should be thinking of making.
"As such, they end up doing some rather bad things...all the time."
Err... what if they refuse? How do you know that they are going to be working for this crime family? How do you know what they are going to do before they do it? My suggestion when planning that sort of game is to recognize that the game can change at any point from the players working for X to, the players working against X. You need to have some contingency plans to handle this change.
You can't really be certain that any given party, including one without righteous paladins, is going to be happy doing 'rather bad things'. Alot of players will simply balk at being forced to do something which they personally in real live would never concieve of doing. It's not unusual for players to be unable to RP a PC which has a morality significantly at odds with their own, and few people are comfortable actively role-playing something that they find distasteful or disgusting.
I would therefore make the assumption that even if your party is a bunch of neutrals tending to evil, that eventually the moral ambiguity is going to present a challenge. It might not, you might have players that reveal in their bad selves, but assume the opposite to be on the safe side.
Personally, I find the campaign idea most interesting if the assumption of a PC ephinany and a change of loyalty is built in. That gives the players a very interesting story arc. I would also spend alot of time talking with the players about their character motivations to find out why they might want to work for a crime boss, and what might make them question that loyalty.