My babies are 13 days old and have already been transplanted once! Since I am no expert, I will have to let others chime in with answers to your other questions. I have learned a lot from reading Marijuana Grower's Handbook, The Indoor High Yield Guide by Ed Rosenthal. My copy is dog-eared from reading over and over.

My reasearch indicates that it is better to start in small pots and get rootbound, then transplant up. However, since yours are already in the 5 gallon pots, I doubt I would move them. Make sure there are lots of drain holes.

There was a post by Nickolaus (sp?) yesterday that had a good ph, soil link posted by StinkyAttic that discusses your PROmix situation. Try finding it by going to Stinky's posts.

Good luck