Mandarins and most wrasses are pretty resistant to ich because they tend to produce a heavy (compared to most other reef fish) slime coat. They aren't by any means immune at all. Not all wrasses need a sand bed, but leopards, tamarins, and those of the haliocheres genus do. (There might be some others I'm forgetting.) You can use a Tupperware bowl with sand in QT to give them a place to burrow. Lined (six, four, eight, mystery, etc) , fairy and flasher wrasses do not burrow.
Wrasses do fine with copper IME if it is properly measured and administered. Many fairy wrasses and flasher wrasses IME are also very prone to internal parasites (more so than ich) and IMO
should be treated with Prazipro for them as well.
As far as prophylactic treatment, that's up to you. I do it with copper and Prazipro (one med at a time for a week each).