Ooh, I have played this game a dozen times. Early game pointer if you don't mind: teach your GFs the ability "Boost". You can earn the points for it easily by running along the beach outside the Garden. It will let you boost their attacks during their summon animations.
Yes, definitely level up your team as evenly as possible. There are multiple times in the game where you won't be able to access your 'main' team. And for the love of everything obtain Diablos as soon as you get the lamp after meeting Rinoa, because he is a bitch to get later if you wait.
When I can find it again I'll link you to the GF relationship chart. Some GFs don't play well with others, and having them junctioned to the same character can lower their attacks. Also, spam the hell out of GF summons. The more you summon them the quicker they will appear in battle.
but cards! they are optional but. if you start with them early in the game, you can convert them into pretty high level magics and trash the heck outta the game
and this guy