Are we assuming that each class has their own job-offshoots? While a plausibility, it's not the only way. They could, for instance, create jobs that combine classes, possibly as well as having some that only require one class. eg:
- Conj only = White & Black Mage
- Thm only = Time & Blue Mage
- Lancer only = Dragoon
- Conj + Glad = Paladin
- Thm + Glad = Dark Knight
- Thm + Conj = Red Mage
- Arch + Pug = Theif
- Pug + Glad + Thm = Ninja

All speculation mind you. Here I go getting abstract and fanciful now:
- Conjurer + Miner + Blacksmith = Geomancer