Use TTM for prophylactic ich treatment on wrasses, no chemicals involved.
I would quarantine 6 weeks minimum, if they came from a medicated system this can suppress signs of disease and it can take a month for ich or velvet to show.
+1
Cryptocaryon (ich) is one of the easiest parasites to eliminate if you use TTM. TTM is simple, effective and exploits the most predictable stage of the parasite's life cycle. As noted above, it doesn't expose the fish to toxic compounds.