OMG! Ruby has grown so much! You must be taking very good care of her. I was 98% sure that Ruby was a female when I sent "her". Now I'm not so sure at all but still leaning toward female.

I think Bogo is a male but some snakes just make me not so sure.

Opinions may differ but I would say it's perfectly safe to keep the two together for now, even if they are opposite sex. Their size differences are fine. No biggie. I've never had any issues with keeping garters together of different sizes (within reason). A 20 inch snake can easily be kept in with babies of only 10 inches in my experience as long as they have plenty of room. Concinnus are hard enough to get them to breed one another,(never had successful matings without full brumation) let alone cross without brumating. Besides that, Ruby is too young to be an unwanted breeding risk IMHO.

Give it a trial run. Watch them closely for any problems and see how it goes. Personally, I wouldn't have any major worries about putting these two together in a 20 gallon. Not so sure about a 10.