Your holster will be fine.
An economical way I know of to address a mag holder for competition would be to purchase one of these from Fobus:
1911 .45 SINGLE STACK - DOUBLE MAGAZINE PADDLE POUCH
For you magazines, since two are 7-rounders and two are 8-rounders, you can get started easy enough by putting the 7-rounders in your mag pouch. Then when you load and make ready with the 8-rounder, it becomes a "7+1", with 7 in the magazine and 1 in the chamber. At that point, all three magazines are loaded to the same capacity and you are legal.
At some point in the future, if you want to purchase a 3rd 8-rounder, then you can load and make ready, top off the magazine, and now have 3 magazines all loaded to 8 rounds which is also legal.
My final point is don't let worries about holsters or mag pouches keep you from going to IDPA and trying it out. No newcomer will be turned away from competing because their holster doesn't meet the rules
so long as it is a safe holster -- which the Bianchi obviously is safe.
Also dedicated mag pouches aren't
required, although they are an advantage. I've had cases where I went to a match straight from work and had forgotten my OWB holster and double mag pouch. So I competed with my normal everyday IWB carry holster, everyday IWB carry single mag pouch, and the 3rd magazine in my pocket. Competing with your normal EDC gear is right within the spirit of IDPA.