plains garters are thamnophis radix
"common" garters are usually thamnophis sirtalis sirtalis
smooth green snakes - opheodrys (liochlorophis) vernalis eat insects and I don't think they're hardy, in fact - I just looked it up for you - they're considered difficult to keep
most radix will take unscented rodents and no, you don't need to tear them apart - it's pretty easy to chop them into strips when they're frozen - think of them as very small steak with the legs still attached - but easier to cut