There are four described species (H. efasciatus, notatus, severus, and spurius) plus many more yet to be classified (may or may not be separate species) such as the Rotkeils. I have seen gold crossed with rotkeil, it happened here when I sold my female rotkeil to a "breeder" who was going to look for a mate for it (they're rare as hens teeth here) but gave up after a week or two and crossed it with a gold and sold them under the b.s line that the "gold gene is recessive so they will look just like normal rotkeils". I've seen a few grown out and they do look similar to the rotkeils (especially when young) but don't colour up anywhere near as well, they don't get a red head at all. I don't think the mouthbrooders have been crossed with the other species/variants, but other hybrids certainly are out there, in fact the common domestic "green" is often a mix and not true to any of the wild types.