what motors etc all depends on the numbers available to race and what they want to run.i think cairns used to run stock for the first half the year and mod or the second.not 100% on that but.
i personally run a 9t brushed in my mod truck in townsville (when i get to race) and i like a 10t in my buggy.i find mod a lot cheaper to run because you dont need the best batteries etc etc to get good performance and checkpoint mods are pretty well priced and last for ages compared to running 27t stock motors.
i think with the short course class there needs to be limits,mainly in keeping with stock type tyres so that it can be kept as a spec style class which would be a great class for beginners to start in.motors possible limited to a 19t brushed for example as well to keep costs down.i have no idea on brushless motors etc which is why i mentioned brushed.
possibly the 4wd slash class could be open motor as it comes with a brushless set up from factory,but still stick with stock style tyres.

when townsville used to race weekends there used to be enough people to race mod and stock buggy(at times there were about 18 stock buggys), mod and stock truck and 4wd as well as a sportsman silver can class.now they race on a wednesday night the numbers have died off considerably.week nights are not a good idea for getting young kids etc into racing due to school etc.