Are you sure? That cars uses different values in one of the three cameras. I don't believe that for this reason that cars can give troubles.Never use cars that have moving cameras(when you turn, the camera moves). Those cars are #1 for causing trouble.
This is possible. This is also given by a low size .ar file, that for the game is more appropriate.I've tried many times to get as many add-ons into MM2 as possible, and whenever I do it, I find that low poly cars (or "crazy" cars) almost never conflict with each other or with high poly cars.
If a car is more similar to the game one's, is more hard that the game will crash for cars conflicts and "memory overlap" problems.