Gained JP Up should be one of the first things you learn for every character.

While the Calculator skill is easily the best in the game, in the meantime as long as your party is balanced with physical and magical characters, you should be fine. FFT is not so difficult that you cannot be successful with most combinations. I always would go for Monks first, and then eventually Ninjas and Samurai (Two Swords is the ability to learn from Ninjas to use for other physical classes). I would, even later in the game, have someone as a Black Wizard just because magic is badass. Just try everything out and stick with what you like.

I almost never used nongeneric units, with the occasional exceptions of Mustadio and later Orlandu. I always prefered crafting my own party from nothing.