You can sell anything you wish. Poems, a short story, pieces of an anthology, anything you want to sell. This is up to you, however keep in mind there is a 2000 word minimum required for ebook publication.
Not at this time. BookRix will consider this in the future, but for now only one author can get paid out at one time.
In addition, the author publishing the written work must confirm that they own the rights to all the published stories. The publishing author must also confirm that they have satisfied any claims in financial and copyright matters of the sub authors.