well garter snakes are awesome pets as i owned like ten at one time.. aspen would be a great substrate but when feeding them you might want to take them out of the cage as they dont eat mice in the wild like corns... they eat worms, fish, which could stick to the food and they could digest that and that would start health issues... but if you start them on pinkies you have to scent the pinky with fish to get them to eat it and then keep doing this for some time and then they usually get used to it... i think for one garter snake a 20 gallon long would be plenty of room as they dont get as big as corns, usually around 24-36 inches... have a heat source too... and 2 hides one on the warm side and one on the cooler side.. you can use an UTH or a heat light which ever you prefer.. hope this helps.. and when you get things set up i would love to see some pics!!!