Hey ace! Welcome to simviation forums

You can do like billerator said, or you can actually install specific files for AI planes and airlines that you want, uinstead of in a package like with Project AI. I like to do it this way, as it's rewarding, more challenging, and you have more say in which planes and routes get flown, but its harder too.
Increasing the volume of traffic through the settings in Flight Simulator only increases the number of Pacifica, Soar, Orbit, and World Travel flights you see, but ProjectAI and the way I do it actually put in [glb]real airlines[/glb] and their planes...
A good tutorial to get you started is
here