[*]DON’T add more fish to the one(s) you already have in QT. You risk introducing a new disease into your QT and having to restart the QT clock. Be patient; add more fish once the current occupants have graduated to the DT.
I don't do copper, so I'm not sure if 1.7 is a lot or a little.
If it's a lot, then shouldn't you gradually raise the copper? May be too much for these new fish to handle quickly. If it's a little then I imagine ich can certainly move to the purple tang and crew. Of course there's diseases that copper doesn't treat some stuff, so if the new fish had intestinal worms or something that could transfer.
My bad advice:
If you have ich in your DT, you have a while before any fish can go back into that system (45 or 76 days depending on your risk tolerance). So I would probably drop the copper level down to a point you can introduce new fish, and then raise it back up. Then do the full gamut of treatment to treat all of them.