Like a lot of people on the forum, I feed a diet of primarily rodents, but supplement with other prey items to add variety to the diet. I do wonder about the affects of a rodent diet on a group of snakes that does not specialize in rodent feeding.
I say specialize, because there are many examples of Thamnophis to do feed on rodents....as was stated before, T.e.vagrans often prey on rodents....I imagine this is due to the fact that they can be found far from what is considered "typical" Thamnophis haibtat where the amphibians and fish that so many others prey on can be found. There are also reports of insular populations of Maritime garters (T.s.pallidulus) that are the primary predators of rodents on those islands. I think that garters for the most part can be called generalized feeders that specialize in what ever prey item is most available for them in their habitat.
As far as garters being able to take on adult rodents, many other nonconstritors take on adult rodents...like racers. Now it's a big garter that's going to take on an adult mouse, since it's got to kill it by just biting it or smashing it into stuff.
I think the problem with possible impactions due to fur and stuff is due to the fact that we feed our snakes bigger mice then they could have possibley ever eaten in the wild. Personally, I don't feed any of my garter snakes mice that are much larger than twice the size of their head. I may have to feed more than one a feeding, but I think this is easier on them. So yeh, there's my two cents on the whole thing (that's a long paragraph for two cents!
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Roy