yeah i agree with Perrin.

From what i've read (probably from the discussions about the latest Dengeki article) If you have gladiator at R50, you can unlock the Paladin Job - however, it would be cool if there are prerequisites Ranks for a related subclass to your Job, for example the Conjurer in this case for the Cures. Thus while there may be a specific class to unlock a particular job. You would be kept busy by being required to level some other classes to unlock jobs.

If a specific class requirement for specific jobs then it would probably look something like:

Job > Main Class > Additional Sub Class/es

Paladin > Gladiator > Conjurer
Samurai > Gladiator > Lancer (Unlock Great Katanas)
Thief > Gladiator > Pugilist (Job will be restricted to Daggers)
Dark Knight >Marauder > Thaumaturge
White Mage > Conjurer > Thaumaturge
Black Mage > Thaumaturge > Conjurer
Red Mage > Fencer > Thaumaturge

Monk > Pugilist > Gladiator??
Dragoon > Lancer
Ranger > Archer


But as you can see, Dragoons, Ranger, and even Monks in this system would seem to be pretty straightforward advancement from their respective base classes.