Will my Glock 36 be competitive? (In spite of the user)
It depends entirely on the user. Why does it matter, were you planning to win the first time out?
With only 6 rounds am I at a disadvantage?
Yes. You will be in SSP (10 rounds allowed) or CDP (8 round 1911s allowed.) So what? See above.
I am not being tough on you. Go, shoot your best with your familiar gun and have a good time. Lots of people put that ahead of hot competitiveness. I consider priorities in the following order:
1. Be safe. Don't hurt anybody, don't scare anybody, don't get disqualified for unsafe moves.
2. Execute the course of fire. Don't get penalized for procedural errors. PAY ATTENTION.
3. Hit the target. Hit it in the middle (or head, when required.)
4. Move along. Your time is your score. But don't run before you walk, LITERALLY. There are three greater priorities.
Also, I can't find on the IDPA site whether mag carriers are required or optional. I have 6 mags but no mag pouches. Do I need them or can I stick two mags out of my other pocket or between my belt and pants?
I don't know what your club will require, but I would let you reload out of a pocket. Some people do carry their ammo there and it is a valid technique, just not very fast. I would watch you closely to be sure you kept control of your muzzle direction while frisking yourself for ammo. I would not agree to magazines stuck in the belt, I do not think that is secure enough for carry.
I have a P7M8 also. I'd rather shoot with my Glock but if I get enough people to tell me that my P7 would be better I'd probably try it.
Shoot the Glock, then. Familiarity counts more than a theoretical technical advantage. But a GOOD squeezecocker man is very effective. And an unpracticed one is a bit dangerous.