My eastern baby was the result of an injured wildcaught female giving birth in captivity. Unfortunately, she was unfit to release because of her injuries, so her owner kept her (she recently passed away from what appears to be old age). The eastern I got from her litter is now 2 years old, and he's doing great! My most enthusiastic eater by far, and very inquisitive. He's out pretty much every time I look over at him. At least compared to my checkered, who's a hermit. >.>
I'd say, if the female can be released, release her. The babies should be fine if you chose to keep them, though.
As far as thiaminase, I'd google some lists. There's a bunch of different ones out there with different amounts of examples on them, but should give you a good idea. You want to stay away from rosy red minnows (often called simply "feeder fish" by pet stores) and goldfish (the most commonly used feeder fish). That said, a purely worm or rodent diet would be preferable to a fish-only diet....Stay away from the red wrigglers, they're the ones that jump around and split apart. They secrete a thick white liquid that is poisonous.
Salmon should be fine, but buy ONLY sushi-grade quality salmon to feed to your garters.
How often you feed depends on the prey source.
Live fish: once every 5-7 days
Fish filet with supplements: every 2-4 days
Earthworms: every 2-3 days
Whole and rodent parts: every 7 days