I think starting the diffuser in front of the rear wheel axle isn't a bad idea, but it would be good to know the exact CFD and dynamic simulation settings (if attitude change will be simulated for example), because this will have an effect on ground effect!
Regarding other regulations, if engine and gearbox are provided, it is again good to have a minimum surface for the air intake, as well as the cooling. For sure on a closed cockpit car the flow is more difficult to control towards the openings than it is for the F1 car, but that would prevent people running fully closed (streamlined) bodies.
Regulations on number of elements for the wings, chord length, location of the wing compared to wheel axles would be good addition as well, unless this is some kind of unlimited category (I don't mind but I think people not having too much free time would be happy to already have sort of boxes where to locate their wing).
Minimum cockpit/windscreen width is a good addition as well, besides the obvious overall length, width, wheelbase, wheel dimensions, minimum ride height, front and rear overhang.