Hey Sigma 40 Blaster,
I'll try to help you out with some of the rules and such for IDPA.
I'll boil 'em down so you can just walk in and they'll approve your rig right there.
First off, the Berreta PX4 is legal under the SSP and ESP categories, start in SSP and go from there.
The mag limits are 10 rounds and as you have seen you just have to load up to ten and +1 in the pipe. Here in Canada that's our legal limit for non LE/Military magazines anyways.
The acceptable holsters are in short, fully functional concealed carry holsters, no dropped rigs (exception for female shooters due to physiology), spaced loops (no light of day through the loops) or barrel forward canted rigs (regular cant with the butt forward is allowed) as these would not be what a person would reasonably use to carry concealed in every day life. Yes they watch closely on this one.
Mag carriers should be something that covers the magazine on all four sides for a minimum of 2" up from the mag lips. Also, you have to do the retention test with the carriers where you have to be able to turn the carrier(s) with full magazines upside down. In order to pass the magazines have to stay in the carrier.
The first shoot usually has a nominal fee, any match after that requires a membership (yeah, they get you hooked with a taste...)
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Some tips to shoot the match...
Best thing to do is to treat each stage like a practice scenario for actual defensive shooting. Use the stage's rules of engagement (tactical order, tactical priority, two to the chest on each, two to the chest on each then return with one to the head due to body armour, etc.) and learn the rules of Vickers and Limited Vickers scoring, not to game it, but to understand what is expected in the stage. Also, practice your one handed and weak handed only shooting as well as draw & fire, fire from retention, mag change and stoppage drills. Make darn sure you practice at ranges greater than 15 yards on a regular basis. Always be safe with respect to yourself and other participants and spectators.
These are things I would tell any defensive shooter anyways. You don't have to get crazy with it (like me). Just be sure you have fun!