First, find out just what kind of mites your chickens have.
Post pics of the scaly feet.
Have you checked them over real well for mites and/or lice? -Google images of lice/mites and their eggs before the inspection so you'll know what you're looking for.
-Part the feathers right down to the skin around vent, head/neck and under wings.
Best done well after dark with a strong flashlight/headlight, easier to 'catch' bird and also to check for the mites that live in structure and only come out at night to feed off roosting birds.
Wipe a white paper towel along the underside of roost to look for red smears(smashed well fed mites).