I've never found butterfly fish to be overly sensitive to copper. I have heard of attempts to cure velvet with Paraguard, without success. I don't trust anything but Cupramine copper to eliminate velvet. A lower dose, than recommenced by SeaChem (they make Cupramine too) is very effective. Bottle says treat at .50. But .35-.40 is totally effective. I'd call SeaChem Tech Support if you have doubts/questions. they are great folks; 1888-seachem, I think.
A bit off topic; but I'm convinced there is a lot more velvet around than we hear about. I think a lot of the ich problems on the forum are really velvet. Especially the fast killing stories.