Copper and Qtank that will be converted into Refugium
 

Have a drilled 20g that I'm eventually going to hook up between display and sump as a refugium. I'm also looking for a Powder Blue Tang so thinking the 20g probably not a bad size for a QT first.

Still reading copper may not be the best for a tang with ich, in case it develops, and hypo-salinity might be better choice, but if I do use copper, could the tank later be tied into a reef system?

Nope, your corals won't do well with even a trace of copper in it. If you have live rock and you dose copper, you will have to throw the rock out as it will absorb the copper.