Garter have a high metabolism. Like a sports car engine. A couple pinkies and a night crawler should be good every 3-4 days. You can see when they have digested there meal as the nice bulge will be gone. Giving a night crawler on a non food days as a treat is fine. Crawlers contain a lot of water and pass quickly. Unless you are stuffing your snakes everyday or so you shouldn't have a problem. Garters will let you stuff them if you want but that is bad for them. Post a few photos so we can see the size of you T. marcianus.