You'll never, ever, ever, equalize all the cars you had in this series, on all tracks, on RS tires, it just won't happen. It would require literally hundreds of hours of testing, on several tracks, by one driver who can tune all the cars and get the maximum performance out of them. If someone who isn't an expert tuner who also happens to be an expert driver isn't in on the testing, you'll never find the true potential of every car. It's been tried many times and it has never really worked that I've seen. There was more than one ringer car in this series, and couple of them didn't see any action. I had a car for GVE in the low 1:06's for example, but never ran it.
Your best bet is to pick maybe 2 to 4 cars maximum, all with the same drivetrain and relatively close weight, and get rid of the RS tires and go to something a little more realistic, like Sports Tires, to bring out more driver skill, instead of just blasting through corners wide open on RS tires. A good example that might work is RX7 vs. RX8 vs Scion FR-S vs GT86. Or maybe Integra vs. Civic. There are a few combinations of cars like that, that can provide good, competitive, close racing. But if you mix FF's with 4WD's and RWD's, you just can't make the cars even enough for good racing, in my opinion.
Not bad plan at all. I'll impose a rule; 4 doors only. Sorry Mazda RX-7 & RX-8, & Toyota 86s, I want some 4 door action. 
