[TheSlyMoogle]

My advice:

Eventually all characters can become all classes. It is at this point that you only continue to pump points into their 3 innates:

Snow - Ravager, Commando and Sentinel
Lightning - Ravager, Commando and Medic
Hope - Ravager, Medic and Synergist
Sazh - Ravager, Commando and Synergist
Vanille - Ravager, Medic and Saboteur
???? - Sentinel, Commando and Saboteur

So yeah. You can max those grids through the main game with little to no issue, but the 3 classes that each do not have innately, they're a bitch to level and aren't worth it.

If you really want to power through the game, go with a party of Lightning, Hope and ????

Also there's a really baller trick to the ATB and Paradigm shifting that makes the game a bit more fun, that I will divulge if you want to know how to do it.