Well you may not want to mix snakes with other species as snakes tend to eat more then just mice, garters expecially. Toads and frogs are something they eat in the wild, and if you put both in the same vivarium eventually you'll be short on toads. :O
As for what to put in, this care sheet http://www.mcwetboy.com/gartercare/gartercare.pdf
will give you a few ideas and guidelines to follow.