I had a couple of Puget Sounds for awhile, the little female was probably my very favorite snake of all time.
But she got an eye infection within 2 days of arriving, not anything I could have prevented, that cost her the eye and I think eventually her life. They were just babies, the female was 3 grams at the time I got them and I was feeding them guppies, earth worms and stripes of tilapia, trout and salmon, which wasn't too hard. The problem with the very little babies is that they can't take even a very small whole pinky. I tried for awhile to switch them over to pinks by cutting up chunks of pinky parts and mixing it in with bits of the other stuff they were eating and that almost worked. But I was having a hard time getting the pinky parts small enough and they would have to struggle long enough to swallow them to figure out, "hey, this isn't fish!" and spit it back out. I only tried for a very brief time, as the gross factor of cutting up pinkies way outweighed any inconvenience of them eating different things than my other snakes.
Most can eventually be switched to mice though. It depends a bit on the species, some are more prone to eating some rodents in the wild and others eat primarily other things.
Viserion has always been a mouse eater, thankfully. He was pretty much full grown when we got him. He does like to go on fairly long feeding strikes at certain times of the year, like right now.
Here is a good general care sheet for garters and the forum it's hosted on is a good one too.
http://www.thamnophis.com/index.php?page=caresheet
In my experience, which is a bit limited, female garters in general tend to make better pets than males, though there are exceptions to that. Garters can either be the most amazing pet snakes out there, smart, (for a snake) interactive and seem to actually like people or they can be the evilest, nasty, biting, musking little terrors you are likely to come across.
