The goal is to have one hundred options, with the idea being that you use only the material you want for your cast. (It doesn’t need to be binary either.) In the over 100 productions the smallest cast has been four and the largest around fifty. The cast needn’t be based on a 50/50 gender balance.
Even the order of the pieces is up to the producer.
As for the songs and ensembles themselves, most of the material is not gendered and not limited to one vocal type. First connecting material such as recitative is minimized. SpDT! is similarly structured but with some twists. SPEED DATING TONIGHT! is what I call a “new numbers opera.” Much of traditional opera is based on musical numbers: arias, duets, ensembles, finales.