Hi marie1975
To treat mites and poultry lice etc. you should be able to buy powder treatment from your local pet store, grain store or vet.
you should treat each bird individually once a week with the powder (make sure you put it under the birds wings, around the neck, and around the vent area, the areas where mites etc. like to hang out)
Also make sure you give the roosts and housing of your birds a good clean and sprinkle the powder treatment on roosting areas also as some mites like to hide in chicken coops (this is important as your birds will become reinfected if you do not do this)
Do this once a week for about a month
I do this when my chickens become infected and it seems to work
As for the no feathers on the chickens front, it may have lice and when treated the feathers should grow back
It may also be missing feathers due to stress or pecking by other chickens if you say it is a rescue bird, the feathers will grow back with time so I wouldn't worry too much
For de-worming your chickens use the one the vet has given you. I haven't heard of giving worming treatment in feed, most require you to add it to the chicken's drinking water
You could ask your vet if you are unsure also
I hope this is of some help
