So... it's a garter snake instead of a rat?
You're asking some questions that have been asked for years and have been addressed on this forum many times before. Until recently, rufodorsata was in the Elaphe (it was considered a type of ratsnake). With recent changes because of genetic mapping (whatever that is), this species was put in a separate genus. It's still connected with the Elaphe, in my opinion, and is not a garter (or any natricine).
He stays hidden all the time. I even had to take him out to get a drink of water... which he took. Otherwise he never moves... except when I take him out to eat. He devours pinks. I will try a fish to see what he does.
One of the reasons I've stuck with schrencki and dione for many years. They are diurnal and active in daytime, and you get to see them more. Your snake could be secretive because it's not diurnal in hot weather; it's stressed from its ordeals; or it is stressed from pathogens. It could be eating lots, but not fairing well, so be careful, and don't over-feed it.
So what is a Rosy red? Is this like a Rosy Barb?
A rosy red is just a common minnow. Try a bait shop to see if they sell minnows.
I guess he's doomed cuz of all the parasites, right? Sniff So how does one get rid of them? My poor little guy. I guess you can tell that I'm a bit nervous about all this now.
This is the main reason I don't recommend rufodorsatas as pets. I don't believe this species is established in the hobby, and thus it's near impossible to find a c/b one. If you try to treat internal parasites, you'll be trying what many experienced herpers have tried before and weren't very successful at. It's been done with many species, such as Elaphe moellendorffi, E. situla, and many others, but with much dedication and perseverance, and species that were profitable for acclimating.
If you try a de-worming medicine, you have to be very careful not to kill the snake with an overdose. Often it's too late anyway. My snake was de-wormed and died anyway after about half a year. Sometimes the snake will be strong enough to pull through. You could take it to a vet, but there aren't any guarantees and that is costly. I recommend you just enjoy the snake for as long as it can hang around and entertain. Be careful not to expose your other snakes to anything. 
PS: You're more likely to get an answer on this forum if you don't ask too many questions at one time, and if the situation is clear with as much info as possible. I still haven't set up my old computer, yet, so I'm not sure I'll be able to retrieve the old pics at this time. Later, TC.