I think Jamster got the closest. If you have a boa, no garter is going to give you many problems. Watching them hunt for fish is also something
every herper should get to do once, anyway. With regard to kings, milks, and rats, they are notorious biters, though for different reasons. Kings and milks usually give a feeding bite, just kind of rolling over and latching on. Rats are known to give some pretty nasty defensive bites. If you're still bothered by the thought of a mildly venomous species, I would advise you to look into small, fossorial species. I can't remember ever being tagged by even a wild snake of that description. Their housing requirements are usually pretty minimal and, besides, worms are
so much cheaper than rats. The only major downside would be the fact that most of those species are not great display snakes. With all of that in mind, I honestly think that the first suggestion given here is the genus you should be looking at.
" a squat, scaly worm with, 'don't touch,' on one end and, 'that's why,' on the other."
-Thomas Palmer