netjunkie had the best response so far. I don't know what rules some of you other guys shoot under, but they don't sound like USPSA to me!
As netjunkie said, you already have a couple of 1911s so I would start there.
With standard 8 round magazines you can shoot the Single Stack class. .45 will score you in major so I would use that over the 9mm unless you are an A zone shooter. If you do shoot 9mm, you can have a maximum of 10 rounds in the magazine (not 9). That is provided you have 9mm mags that will hold 10 rounds! They can NOT be modified. Another advantage for you is the holster required for Single Stack must conform to IDPA rules. You can NOT use a race holster or a drop offset holster. Single stack will also require you to keep all magazines behind your centerline. Magazine holders must be the IDPA type and not race type. All this means to you is that the gear you have now is perfect for this class.
With your standard 1911, you can also shoot in Limited 10 if you buy some extended 10 round magazines from Chip McCormick or Wilson Tactical. Limited 10 class would allow you to use a race holster or a drop offset holster. You can also mount your mags anywhere on your belt and use race type mag holders.
Between the two, I would start you in Single Stack if you were coming to a match with me. After you shoot in 4 or 5 local matches and had a chance to see what other pistols are being run in what classes you will have a better understanding of where you want to be for the long haul. You may decide to shoot multiple classes or you may decide you like the Singe Stack class the best.
On that note, I shoot Limited with a .40 STI Edge. With very few exceptions, everybody in limited class shoots an STI or SVI in .40. Some people do shoot Para, but they are few and far between in Texas.
An Edge would be a bad choice for Open class unless you had it modified. Open alows compensators and optics. You can also make major with .38 Super and 9mm. A better Open gun is something like an STI Trubore.
Most of your questions can be answered in the USPSA rule book as for what is allowed and what is not. The simple break down is this:
Production is for 10 round striker fired or DA/SA pistols with no mods. No single actions allowed.
Single stack is for factory 1911s with no extended mags and minimal mods
Limited 10 is for 10 round single stacks with SOME mods
Limited is for double stack mags with capacity higher than 10 with SOME mods
Open is for full blown race guns with comps and optics
Revolver is for revolvers.
I hope this helps and Welcome to USPSA!