If you're talking about using Ivermectin, I know a second dose in 10 days is recommended for worms, not sure about mites. Also be careful with ivermectin as it's toxic to birds in high doses (i.e. if you miscalculate the dosage you can kill your birds). I think your main problem is going to be killing the mites in the barn... They're good at hiding in all the cracks and wood joints and live a long time. Frequent treatment with permethrin or something might be necessary. Finally, a bad mite infestation as you seem to have can leave chickens anemic. If you can get them a vitamin supplement I would. Particularly for your silkie I think rooster booster would be more beneficial than the vetrx at this point.