It's easy to get most jobs up to rank 20 or so due to the decreased xp requirement in that level range. However why did you reroll? You can change jobs and even your base stats at any time. If you are interested in crafting however it would be helpful to rank up Alchemist and Carpenter to 10, to obtain Preserve and Tender Touch. These are abilities you can set that can help you maintain synthesis stability (Preserve) or lower durability loss (Tender Touch).
As you rank up the xp requirements will steepen and you'll need to specialize in one or two classes to get them to max rank in a decent amount of time. They also plan on retooling some of the recipes as to not require so many items, many of which pull from mulitple crafts you may not have leveled. With the current setup however, say you want to be a weaver it's a good idea to rank up leathercraft as well ad you'll need items from both professions to effectively make gear.
As for gathering classes if they interest you these classes will help you obtain raw items to support these crafting classes as follows:
Fishing: Alchemy and Culinarian
Botany: Carpentry, Alchemy, Culinarian and Weaver
Mining: Armorsmith, Blacksmith, Goldsmith (and a few Alchemy items)
Leatherworking primarily relies on drops from mobs throughout the game.
Classes are meant to work together but as you rank you'll need to narrow down a specialty unless you have nothing but time.
As for battle classes, pretty straightforward, kill things and rank up. Rank Pug to 6 for Second Wind and get a few levels in THM/CON for Sacrifice or Cure and you'll increase your survivability greatly. Hope that answered some of your questions