What timing system will you be running? Will it be AMBrc so we can use our personal transponders?
My opinion on classes is that you should run the following:
Tamiya mini
Novice (with 540 motors only)
540
27 turn Stock (and 17.5 brushless from 1 July)
19 turn Super Stock (and 10.5 brushless from 1 July)
Mod (if the track is big enough)
12th scale (maybe as I suggested with a 19 turn motor limit)
Drift
Buggies on asphalt is not such a good idea as off road tyres are very soft and don't last long on asphalt.