Source: http://www.ffxivguild.com/ff14-job-requirements-arr/Note: (Blue has been confirmed by people in the Free Company, or someone who posted here)
Additional Sub Job SkillsAdditional supporting classes can be used for specific jobs, I have not confirmed the lists yet, but if you know that data as well, let me know and I will add it.
Obtaining the “Soul Crystal” of the Job requires you level up your main class to 30, and sub-class to 15. Your guildmaster will direct you to your Jobs “Guildmaster” once the requirements are met!
ConfirmedBard / Paladin / Warrior / Monk / Dragoon / White Mage / Black Mage / Summoner / Scholar
Advanced JobBase Class (Level) -> Secondary Required Class (Level)
BardArcher (30) -> Pugilist (15)
PaladinGladiator (30) -> Conjurer (15)
DragoonLancer (30) -> Marauder (15)
WarriorMarauder (30) -> Gladiator (15)
MonkPugilist (30) -> Lancer (15)
ScholarArcanist (30) -> Conjurer(15)
SummonerArcanist (30) -> Thaumaturge (15)
Black MageThaumaturge (30) -> Archer (15)
White MageConjurer (30) -> Arcanist (15)