Welcome. Inbreeding hasn't shown to be a problem at least in the short term. Cross breeding does not work.
Hello, Rick. By the "biological definition" of species, two separate species can rarely interbreed, of course. But subspecies should occasionally be able to have mixed offspring. I understand, for example, that the Chinese mantis has at least two major varieties. As a kid, I tried to get (what I believed to be) these two varieties to cross, but with no success. This perhaps indicates that these two "varieties" are in fact separate species. But order mantodea, as I'm sure you know, is even richer than our own order primata, having somewhere around 2000 species (correct me if I'm wrong). Yet many primate hybrids are known, both in the wild, and especially in captivity. Altho insects seem to have a greater number of mechanisms to prevent hybridization, it should still be possible to find two closely related species or subspecies that can yield viable offspring. That it hasn't been observed by breeders might indicate that breeders are getting their animals from narrow stocks, and that there aren't any closely related sub-species to select from. But as an experienced breeder, maybe you can tell me just how diverse the breeding stocks are for each species. For example, are most orchid mantids in america descended from just a few breeding pairs? If that's the case, then you wouldn't actually have a variety of subspecies from which to breed.
As for inbreeding, loss of fitness, especially WRT ability to survive in the wild, is not something readily visible. Zoo breeders are very concerned about turning there animals into domesticated species. An constant influx of genetic variety is the solution.
BTW, are you familiar with "zoovival"? It was a project to use private breeders to preserve endangered species. I can't seem to find any info about it. I figure that you might have heard of it.