I would advise you to focus on one job till about level 47-50.

At level 17 you unlock Palace of the dead which gives decent exp but it can also be mind numbing if all you do is spam it and at 47 you unlock squadrons (if you leveled you Grand Company) which you can take into dungeons which also give decent exp.
You also unlock beast tribes at 50 i believe.

Until then you're stuck with fates and daily roulette things.

edit: there are alternate ways of leveling your lower level classes but it requires you to have 1 of them at higher level which is easily done with MSQ experience.