Awhile back, I asked my vet about using Frontline on chickens, and he said it should work and be safe. When I had a mite problem several years ago, I used Ivermectin (and it works for de-worming, scaly leg mites and NFM). Frontline works on the surface of the chicken like it would a dog, Vet told me. I have not had a mite problem for several years (knocking on wood), but unlike some other areas of the country, it has been unusually dry here this summer (drought-like) so not very conducive for mites.
IMHO, I also found, several years ago, that straw (as in hay straw) helped harbor mites. I eliminated the use of hay in the coop and used pine or cedar shavings instead (as needed).