As u may know you can start out with 6 Jobs to choise from, leving them up and subing others to get some strengths from them. To get Jobs like Paladin, Ninja, Summoner, Dragoon, Samurai and so on, u go on quests, not just lev up these and those jobs and this one becomes one to choise(like FF Tactics).
To answer some of your other questions, when starting out in low levs, it's better to focus not only on the job your working on, but jobs that are like it. Like, if u want to have magic and still do Damage in melee, start off being a War and then later on become a RedM/War. Maining RedM will give the magic your looking for and subing War will give u more Vit and Str. If your going to go this route though, be hume not Elvaan. This way the Wht and Blk magic will be balanced and u'll have better MP(though War will not help the RedM in that area).
Also, about Paladins, they're the best tankers in taking Dam; that I know of. I think Elvaan Paladins rock with War sub(lots of Dam, provoke and can take the hits). Being Elvaan and subing War though will make it so you don't have as much HP as a Hume or subing WhtM in the long run.
(Note: Both the setups I've talked about are pretting good for soloing, but better for PTing).
Lastly, when you get into High Levels, I believe you can Main and sub almost whatever u want and do ok(long as they're not too apposing Jobs). It's just when your at low Levs u can get your rear kicked real fast vs being high Lev, Leving somewhere safe for it(not many safe places at low Levs).
Hope this helps u and anyone else that runs by it.
