The Interex appear in the first Horus Heresy book, Horus Rising as a branch of humanity heavily influenced by eldar.
I was forever saddened to hear they never made it.
Do any rules official and/or unofficial rules exist for playing as these? Also do any models/model making guides address making an army of Interex?
I would think either rules similar to the Eldar or Tau would fit, but I have no idea if that would work.