You get materia from convering gear (where reads convertible, some say non-convertible) when the spiritbind reaches 100%. You shouldnt worry about them til you reach level cap, since you will be replacing your gear quite frequently. Besides you would need to find crafter to meld for you, unless you dont go and level up them yourself.

I'd get CNJ to 15 as soon as possible, there are no benefits of getting one to 50, plus the skills you get from job quests at 30,35,40 etc requires you to have PLD unlocked. Makes everyones life easier when u get shield oath & other fancy stuff.
You get 50% more experience points to every class below your highest, so lets say u got GLA lv50, you get 50% more exp to every class.

No clue how it was in wow, higher level lets you equip higher level crafting gear & tools so you will have higher Craftsmanship & Control, meaning you can craft better stuff basically. Getting most crafters to lv15 helps, since you can get their crossclass skills.