There are barely enough quests to get your through one job. You can use the low level ones in each city on a class from that city. So, say you start as CNJ in Gridania, do all the low level quests on CNJ. Then when you go to Ul'dah and unlock the classes there you can use the low level quests on one of those classes. Same when you go to Limsa. But by the time you start doing dungeons there aren't enough quests to spread around to multiple jobs. I was having to spam FATEs sometimes just to get another level to unlock the next series of story quests. The quests you get from 47-50 give a lot of exp, and you can get to 50 just by doing the quests and jumping in on any FATE you run across. I actually hit 50 and still had a few quests in Northern Thanalan that I left undone. Since you stop acquiring exp at 50, I'm not going to waste the exp. I'm saving them for the next job I get high enough to do the quests. But for the most part, from 15-50 you'll probably be doing all the quests on one class/job. If you want to level that many jobs simultaneously, you're going to be grinding a lot of FATEs.