Except SWAT, teams are balanced fair enough. I don't think we need to complicate the gamemode by including additional classes.
In most cases president dies so fast not because he lacks protectors. They are just hanging around somewhere, camping at armour or hunting particular terrorists. Or, what happens more often, the president is just some newbie who has no idea what he is supposed to do. I don't know how change that btw, nobody would read no tutorials/instructions if you put them on, say, unregistered president's screen on spawn.
But, anyways, you can add 10 more classes, that wouldn't change anything, but that will confuse newbies and probably annoy some regulars. So, following the old good KISS ("Keep it simple, stupid!") principle, I say -1.
As for SWATs, they definitely need to be reworked (or removed). I like the idea of giving them two-handed weapons only like M4, MP5, Desert eagle, that can be attractive.
