Only time will really tell with your garters. Remember that garters are very active snakes, so they're not as apt to just kind of "hang out" like boas and cornsnakes. This sometimes means that they can be a bit more flighty and possibly nippy. My little Petra (female T.s.concinnus) is really great once you get her out of the tank. Doing that usually involves a lot of biting and a little blood (she's only about a foot long now). She's just an aggressive feeder and bites anything that moves in her tank. Once she knows it's not feeding time, she really mellows out and just kind of cruises around. Some garters can be really mellow and are a lot like your other snakes, then there are others that make mambas look down right cuddly! Just differs a lot from individual to individual and especially between captive bred and wild caught. Best thing, just enjoy your garters for their uniqueness and maybe with time they'll mellow out..............some.
Roy