1% Ivermectin injectable dose for poultry is 1mg/kg or 0.045ml per pound of weight. (0.23ml for a 5lb bird). Injectable can be given orally, topically or by injection. Dose once, then repeat in 10-14 days.
Ivermectin will probably be one of the quickest ways to get on top of mites on the body and SLM as well. It won't be that effective against lice, so you may find that you will need to give them a light dusting of Permethrin poultry dust IF lice persist.
Do slather the legs with some oil (castor oil, coconut oil), vaseline or Nustock cream to help heal the legs.
If you have photos of the birds and their legs that would be good.
If they have been in poor conditions, take it slow. See that they are drinking well and provide them with food free choice. Make grit (crushed granite) available as well.
You may need to treat for Coccidiosis once they have been on new ground for a week or so and it may not be a bad idea to deworm as well. Just depends.
Do keep them housed separately from your existing flocks until you have been able to get parasites taken care of and have had time to observe for any other symptoms of illness.