As far as crafting and gathering goes, I'd advise leveling them all up evenly. The way the system is designed, the items you gather at level 20 will be used in level 20 recipes to make level 20 gear. Also, crafting HQ items (the main reason to craft) is MUCH easier through the use of cross class abilities. Finally, until level 50 all your crafters use the same gear, and all your gatherers use the same gear.