If you get a chance, watch a model yacht world or national regatta. We had one recently and it was amazing. Short races, extremely tactical, while still being speed oriented. Finding the right balance between being conservative and aggressive was very important, too conservative and you'd be out the back door, too aggressive and you were quickly taught a lesson in boat speed.

I wouldn't advocate short races exclusively. A regatta format could be a short race or two followed by a standard race per day.

Short races for cats could easily be windward returns, finishing downwind on a standard length leg. It'd be over in about 10-15 mins.