Steve,
I need to apologize for just seeing your reply right now. My reply may be a little long, so bear with me...
I started my AI when I had FSX, and that was with WoAI - now a little long in the teeth. From there on I have gone on to collecting traffic from mainly AIG and MAIW, but also from wherever I could find traffic, for instance on AVSIM and on this august board.
Every flight plan I find I run through AIFP to ensure validity and compatibility.
The AI aircraft also come from the above sources, but here is a catch - in P3D, the "model" (the "basic aircraft skeleton) HAS to be at least an FSX model, FS9 models will not work in P3D as they will in FSX. Even better is of course native P3D models, and the AIG website has a lot of FS9 models that have been converted to P3D.
But - there is always a "but" or two - AI traffic is no good without airports where the AI aircraft can land and PARK especially. Here you have to make sure that any add-on airports are P3D-compatible, and also compatible to your specific version of P3D.
Hope this makes some kind of sense -
Jorgen