I don't really know much about it, but it sounds like you can add custom AI traffic but the aircraft models must be in FSX format or later, and the traffic files should be in FSX format for maximum accuracy.
I'm a bit late to this party so apologies for resurrecting a dead thread: Does anybody know if AIFP is compatible with P3D 4.5? I only ask because on their site is a ss and in that ss it says "Target Version" but it only has "P3D" and "P3D v2" (along with the FSX choices). If it's compatible, I'd use "P3D v2" format?