To me, very slim. I've seen other species trail spawners to eat the falling eggs, but never get involved. We see it a lot more with livebearers, because you generally don't have to do anything special to spawn them, and Cichlids because we keep them in communities and we see when they pair off and who they pair with. A few rainbow species have been made extinct in the hobby here by lazy or thoughtless breeders, as they cross very easily.
With tetras, usually you have to set up a single species tank to breed them, and you would have to be clueless to put the wrong species together. Plus, you'd need species where the separation wasn't that ancient, so the fry would be viable even for one generation. If it happened easily, we would have 2 or 3 tetra forms available by now, with everything going into the soup. In the hobby, hybridization is a great enemy of diversity.
If we want hybrids, the farms will find a way. If there's a profit to be made, it will be. But other than the mess made of rainbows and livebearers, we've been pretty lucky. Even why hybrids survive, they are often sterile mules.
