Personally the way I set up the jobs in XII is as following

Basch-Knight (no magic, heavy armor, shields, swords, greatsword, break techniques)
Vaan-Thief (no mage, light armor, dagger, shields, ninja sword, and steal and gil toss)
Fran-Archer (first couple green magics, heavy armor, bows, poach
Penelo-black mage (Black magic, mystic armor, staffs, and no techniques but libra and charge)
Balthier-gunner (no magic, light armour, guns, and horology)
Ashe-white mage (white magic, mystic armor, rods, and no techinques but libra and charge)

latter in the game I'll upgrade the classes.

Basch- Dark Knight (Arcane Magic is added)
Vaan- Sky Pirate (Time Magic, swords, and shields are added)
Fran- Hunter (Green Magic, and Spears are added)
Penelo- Sorceress (All Magic is learned, Katanas are added)
Balthier- Sky Pirate (I usually just wing it with Balthier)
Ashe- Paladin (sword, shields, and sometimes time magic is added and maces)

This is just a general idea on how I do it, sometimes I'll make small alterations, but I try to stick to the this formula.