Unlike XI, Your sublcass you can use all their skills up to their own max level however when dragoon at Cap your limited to like 5 abilities from your subclass. You do not have to level them to cap but need the minimum of lvl 15 to unlock dragoon.
To be the best you only need to level your subjobs to get the sublclass jobs abilities. However in this game its best for you to have all classes to 50 to be able to exchange rolls as needed to complete content.