I'm glad I stumbled upon this post. I was going to ask a similar question but not about crossing different thamnophis species. That's nuts.

If all the various subspecies of Thamnophis sirtalis are indeed the same species, then there really shouldn't be an issue with breeding (even if it has to be done artificially) say, a s.f. garter with a concinnus or ca red-sided? Afterall, there are pictures on the web of snakes that appear to be the result of two different subspecies producing offspring together.