Kyle and Liz, I will attempt to answer your questions but hopefully others will chime in as well.
1) Yes, now is the best time.
2) Rule of thumb for me is hole needs to be twice bigger than the current rootball diameter. depth would be the same as your current depth.
3) You can use regular garden soil but mix it with the natural soil. Hopefully your soil is sandy and well draining. Plumeria does not like wet feet.
4) If it were mine, I will plant it in the center of the hole and plant it as straight as I can possibly make it. I tried to stake and straighten a plumeria this weekend but it was severely crooked that I ended up breaking it.
