Definitely ARC (Archer) for the BLM soul quest. Don't worry, ACN wasn't wasted as that is your other set of cross class skills, of which Virus is a debuff and Eye for an Eye adds to our defensive toolkit.

I haven't been able to settle on a rotation, but I don't think its as important considering how much we may or may not have to do besides dps in a fight. I figure most of this will be for Umbral state, like healing or def/debuff stuff, then Astral for dumping damage. Transpose could be more of a handy tool than a necessity if BLZ 3 and Fire 3 swap so easily.

I can say, for those just getting started levelling, CNJ to 12 gives you cure, protect and raise, all super handy and not hard to do at all. I love having protect for when I'm out solo levelling. I don't really use Blizzard anymore unless I mess up my u/a or forget to transpose.