Okies,

You can specify a particular ai to join the rcae using "/ai [name]", but is there any way to make an AI select a different car to the user who invokes it (like an XFG if we are running TBO class).

Nope, it joins with the car you are on, in the pits.

Select the car you want the AI to use and put him on track, then return to pit and select the car you want and join the race.