There's two varieties that I've heard of, but I don't think outside of the original farms no one knows for sure.
The first is the smaller variety, that are a convict/severum cross. The other that I've seen, called the King Kong variety are supposedly Midas/RD/Severum crosses.
I'm sure there's all sorts of other things mixed in there too besides some crazy inbreeding, but those were supposedly the main ingredients.

FH can cross with Parrots, I have two small ones (not really fry anymore) that I have no idea who the parents are. One is splotchy but the other has fairly pronounced stripes, and the only adults remotely striped in that tank is a lone flowerhorn.