just throwin out ideas that I have done in past frag growouts. Maybe grab two different colonies (one zoanthid, one paly) and give each participant two frags, one of each. Do an overall best growth on both types, then best zoa growth, and best paly growth.
Or a similar option, do one group using paly frags for growout, one group using zoa frag for growout which will work in the case of a large amount of members participating.