I think that the treatment advice offered by KnaveTO will probably be an excellent choice. However, and I am not trying to be nasty here, please invest in a quarantine tank. A well prepared 10 gallon aquarium would have pretty much eliminated this entire problem - and it would have cost less than that single plecostomous! I imagine, sminker, that you have already reached that conclusion but this advice seems to be worth repeating in light of the number of times this same scenario has been described in the time I have been visiting this site. I had to hear the same advice more than once before I joined the choir but now I am a firm believer and quite happy that I made the decision to use a quarantine tank before I started stocking my 410. I have also decided to use a one month minimum time in quarantine just to be safe. A bit extreme? Maybe, but let us know how you feel once this issue runs its' course. I'm betting that once you figure in the time, money, effort and aggravation you'll agree with that old "ounce of prevention..." thing.
I sure hope that all of your fish survive this ordeal!
Beasts