Killfish, they are more common around saltwater areas cause there more meater and fattyer for bigger fish , ive noticed that alota freshwater fishing shops have shinners or shad or standard american minnows .
i would go with the killfish , they get rather big and there very soft, no sharp big scales or spiny fins or anything that would cause a problem and the killfish eat like champs so if you raise them yourself you can fatten em up with all kindsa nutritional items that would be ideal for garters.
also if you go fishing then in the spring and summer u can slice up sunfish or small trout or something abundant that your allowed to keep alot of that you probobly wont eat yourself and just fillet them and slice strips of meat and theyl even take the hearts and guts and just about anything you feed them (no bones or sharp parts obviously), now in this case if you feed them the strips they wont get full nutritional value since there only eating a certain part of the fish wich could lead to vitamin deficiancies but i only do this as a treat for them here and there , but they love it .
thats about all i have to share about that
i have a question myself, waht exactly are the red compost worms ? are they the trout worms you get at fishing shops?