One other option, is creating two collab stories together that mesh together cohesively in some way. Maybe a two part story, or two different perspectives of a relationship. That could lead to many possibilities. Pairs of two, create a story of their own and then the take turns to edit and offer advice for improvement. In that way, no one feels like they not pulling their weight or contributing enough to the story itself. And gives well needed skills to give and receive critique. Also it would allow another smaller word count and still give a little more substance. 

And yes, the rules need to be more precise. On the premise, wordcount and whatever else may be applied. So 2/3rd's of people don't completely disregard them and ironically those that followed the guidelines probably 'suffered' for it.