Hibernation season is an entirely different issue.

Just prior to brumation ("Hibernation") snakes go off feed, they stop eating until spring arrives.

So a six foot milk snake that would otherwise eat a small garter without hesitation will completely ignore the same snake over the winter.

This is how communal dens are possible.

Never house mixed species if one is pregnant (Gravid) for instance an adult garter snake will not eat a Dekayi or red belly of similar size. however that same garter will consume a whole litter of these at birth and not think twice.

Mixing corn snakes / milk snakes / and other Colubrids with garters is not something I would chose to do and will always have a bad ending.

However mixing sub species of same sex garters is common, and works fine.. in other words it is OK to keep a radix with a sirtalis of similar size.

Small male garters seem to get along fine with larger Dekayi snakes in the same tank, We have one 30 gallon with 2 small male eastern (Sirtalis) garter snakes and 3 large female Dekayi snakes, and they have lived just fine like that for years.