btw, thanks for all the advice and help. ohh and the two plecos don't care one way or the other about the Cardinals and same for them.
In my experience, bottom feeders tend to have more impressive immune systems than column fish.
Cycling the new quarantine tank isn't absolutely necessary, but you'll have to watch ammonia and start doing daily water changes after the first couple days. You can also take some of your filter media and either throw it into a new tank to jumpstart cycling, ideally inside a filter. You can use the 20g, or buy a 10g for $20 or so at your PetSmart. Hopefully you have a spare filter? If not, a cheapie will work fine.
At this point, not sure using a quarantine tank will do any good. You might want to wait it out and hope for the best.
I have a 10g that sits on my kitchen counter for QT. No stand necessary, and I keep disposable plants (some moss and ferns) in it, as well as a few BA tetras, which aren't terribly plant friendly. It's got the stock hood, with mini CFL's replacing the incandescents that came with it, and a couple small filters. Anyway, I keep a few things in the tank to give it some charm (heh), and I'm not going to cry over anything I normally keep in there. All new fish go into it for a month, unless there's a death, and then the month cycle restarts. It's probably a total $50 investment. That's one easy route you could go.