From what i recall of the rulebook is that the holster has to cover the trigger guard and keep the gun secure while running, walking, basic movements like that. And your mag pouches are required to secure your mags while walking and running. It's been a while since i've shot USPSA or read the rule book but just get what's comfortable for you and the RO will let you know when you go to get your certification (pretty sure all clubs require you to show proper equipment and ability to safely handle, shoot, reload, and move with your firearm prior to shooting your first match).
Good Luck and Have fun.
PS check out
www.uspsa.com they have new shooter info on the site