What snakes can you put in with your garter snake?

None. Garter snakes dont get very big so if you put any other snake with it, the garter snake might get eaten.

Actually, nothing besides a kingsnake would eat a garter. I would say don't do it though, because of space and basking competition, and diseases.

There are many snakes that feed on other snakes not just Kingsnakes so you have to be careful on the species that you put in the same container, Indigoes, racers, cottonmouths, coral snakes, whipsnakes are some that come to mind>