If you are gonna play more than one class, your first 3 should be one of each role imo. Tank heal and dps. Try to level a dps to 50 first and absorb all the xp reward from story quests. Your tank and heal can lvl through quick dungeon instance queues. Avoid finishing minor sidequests that are below your first dps class who is going through story. Your other classes can use those xp unless you want to be left with just fate grinding and long dungeon queues for xp. So if you hit lvl 33 and the zone has lvl 31 32 quests,leave aand save them for later.