I highly recommend doing all classes on 1 char. You can use several cross-class abilities to make your main class easier. When you get it turned into a job you're restricted in the cross-class skills though.

The lower level classes get bonus exp up to where your highest job is. So if you have 1 job 50 and start a lvl 1 job I think its a 100% bonus exp increase now (number might not be right).

Also leveling low classes/jobs just with hunt log/fates/leveling roulette/dungeons/guildhests/levequests goes by pretty quickly. Don't need to worry about running out of actual side quests.