Same here, I feed my garters worms, crawlers, fish, newts, and for a treat small toads.
Many will frown on this practice, but I have had garters for years and never had a problem.
Regular fecal exams are a plus, especially with a diet like that. Most vets will look at poo for around $30.
Never had any luck with mice, but my snakes are healthy and keep on growing, so something must be right.
I feed the same items wild garters eat every day, so how can that be bad??
For animals that are not supposed to be social, my snakes seem to get along just fine. (within reason, a big snake and a little snake together spells one less snake to feed.)
Regards,
Wayne A. Harvey

our critters
-----

1.2.9 Storeria dekayi Casper, Xena, Athena, & Kids
0.0.2 Thamnophis marcianus - Checkered Garter Snakes
0.0.1 Thamnophis sirtalis - common Garter - Princess
0.0.1 Nerodia sipedon - Water Snake - "Aqua"
0.1.10 Storeria occipitomaculata - Red Belly snakes
0.0.1 Amelanistic Corn Snake "CY" Juvinile CB
0.1.0. Pueblan Milk snake "Oreo" adult CB
1.0.0. ASIAN GREEN SNAKE 3' WC Cyclophiops major
0.0.1. Savannah Monitor "CHOMPER" CB
0.1.0. Green Anole "Crystal" WC
1.0.0. K9 "ACE" Black Cockapoo
1.2.0. Feline"Felix" "Kaja" & "Silver"
0.1.0. calico RAT
2.4.?? Mice - Feeder farm - Crickets / fish
More herps than I could ever list out back on the land. 
