I just leveled my 2nd class to lvl 15 to take on my Job class. I found it difficult to find supplimental quests to do levelign through. I ended up doing most of my leveling through Fates and Leve's.

So keep that in mind, if you're a quest type of person do what you can to leave as many alone as possible but just be prepared to do some grinding and fates when you do additional classes.