Do you see evidence of feather mites on the silky? 15 days old is quite young to treat with Ivermectin, imho, but if you are very sure that she has them too, then treating her to get rid of the mites would be good.
A vet should be able to examine the feather quills under a microscope to see if mites are present so that's another thought you should consider.
Some folks have good success ridding their birds of the mites after treatment, but some have found that the mites are persistent so retreating quarterly or twice a year may be needed.
While feather mites do live on the bird under the quills, they are like most parasites and can migrate from bird to bird so treating housing (Use your Permethrin II) for that would be good. Make sure you treat roosting bars, cracks/crevices, nesting boxes, etc. Take out bedding and replace it.