Need clarification on classes and crafting. Today I joined the carpenter guild and was wondering if it was possible to join each guild in relation to crafting/gathering. Also say if I want to build a chair as a carpenter, can I actually gather the needed materials as a botanist and than craft the chair as the carpenter? Would I be required to rely on other players (aka the market) for certain items to build/craft/gather. I ask because I'm trying to figure out how it all works out. Could one simply take up every crafting/gathering class and take the time to gather the materials to build items himself and not rely on the market say?
Oh and how would one level up each class if you've already completed a bunch of quests as say an Archer at the beginning other that killing monsters or entering FATE's. Is there an alt method of earning XP?

Clearing up my confusion would be greatly appreciated friends.
Thank you.