You can breed any subspecies of garter together that's in the same genus. For example you could breed an Eastern Garter (Thamnophis Sirtalis sirtalis) with a Red Sided Garter (Thamnophis Sirtalis parietalis). The reason why most people don't do this is because you get a hybrid of the two, unlike when you breed morphs together and can separate them again into normals and whatever the morph is.