Hi Hjelte,
It's no problem to keep more garters together, they're definatly not teritorial ! But I recommend, like you already said, take a garter that have the same lenght. And if you want a different sub species than I think it's best to have the same sex as your own plains. You be suprised how many sub species are mixing together !! I think nobody wants that. But if you want to breed radix than it's nice to have the opposite sex (radix ofcourse)
Any member of the sirtalis group should be fine with radix. I wound't put a cyrtopsis together with your radix, becouse cyrtopsis are known as difficult eaters and radix are probebly the best, so maybe the cyrtopsis will get stressed than.
Good luck and let us know what you decide
Sabine