The T-Shirt/Polo cover garment is fine. The only requirement is that you can't see the gun/mags when holding your arms out. Do make sure when you pull up your concealment that you don't get your support hand close to the muzzle. Also make sure you can holster the gun without assistance from your support hand. But no local club is going to prohibit you from shooting for the first couple times if you don't have the right kind of concealment or equipment as long as it is safe. However, make sure the holster is a strong side holster (IWB/OWB) which holds the gun so that the trigger is centered or behind a plumb line from your armpit. For a cheap double mag pouch look at the blackhawk series: https://www.amazon.com/BLACKHAWK-Dou.../dp/B004T0YSBO.
Most people use OWB holsters. However, lately I have been running a Comp-Tac IWB Kydex that is just as fast as the OWB version IMO. http://www.comp-tac.com/inside-the-w...r-10217?page=2
I don't have any experience with IWB mag carriers, but I never see them used.
Enjoy and be safe our there.
Cody