Having less dissolved organics, fewer particles in the water column and lower nutrients will help. But it's likely their population will self regulate at some point due to the above. Unless your system has a lot of all those.
Six-lines can be rugged with other wrasses, though those added before the six-line will often fare better.
Copperband butterflies are reputed to be murder on feather-dusters, as well - I'd be surprised if yellow longnose butterflies wouldn't decimate them too.
Copperbands are .... mostly harmless. They'll devour featherdusters, will sometimes pick at the mantles of clams, and occasionally cause trouble with some of the LPS.
They're also known to snack on Aiptasia, so more than a few reefers are willing to accept the risk.