Not quite
You're partially correct. The rules of the International Defensive Pistol Association (IDPA) do not allow appendix holsters. The rules of the United States Practical Shooting Association (USPSA) allow the holster to be placed in the appendix position if you choose to compete in the Open, Limited, or Revolver divisions.
Absolutely not. You must have certainly misinterpreted whatever you read that led you to believe so.
You should go and observe some matches first, and while you're there ask the participants questions, before you think you understand all of this.
During the USPSA match I competed in two weekends ago I noticed a guy hanging at the back of my squad observing. I just went over during my downtime, introduced myself, and told him he was more than welcome to approach, observe, and ask. I answered as many of his questions as I could and gave him some basic advice on how to get started. I do believe I'll see him again, next time with a pistol on his belt.
Find them here: www.practiscore.com, and here: https://uspsa.org/find-a-club