Wait for someone more experienced than I, but I will give my 2 cents.
If the pleco is OK in the 10 gallon size wise, and I think a rubbernose would be, I would give him rounds of Prazi in the 10 gallon. While you do that, do the Prazi/salt rounds for the other fish in the 55 (assuming you have the other fish or are getting them soon). This is assuming he is not showing any symptoms that would require salt or other form of medication.
Keep in mind that it will be less prazi and less work with the WC's in the 10 gallon tank.
If you are going to be spreading out your fish purchases you will want to do seperate QT's anyway which makes it a bit of a moot point. QT each fish in the 10 gallon as you get them which will also give your filter time to adjust to the additional bio-load