Take note to the last sentence in Robert Rays clarification.
Answer: If the gun fits the box with the magazine inserted then it is legal to use. They still have to comply with the division criteria for max rounds in a gun and if the magazine holds less than division capacity they can not mix mags to suit the stage. If they use a nine round magazine, then they can only load nine rounds in all their magazines even if one will hold ten.
International Defensive Pistol Association
Note the part in bold. I have noticed shooters combining 7 round magazines with 8 round magazines in CDP. This clarification tells me that doing so could result in a FTDR if if done intentionally to gain a competitive advantage on a given stage. At the very least a PE if accidental. If a stage had one popper and 3 targets, go to slide lock vs a forced reload with retention it would appeal to some to use a 7 rounder on this style of stage. Watch out Ill be watching you. <jk>