What age can you treat chicks for mites/lice etc?

I have a two and a half week old chick who has been scratching on and off for a while.  I wondered if I could use the mite/lice powder on her?  The powder I have is the Barrier one.  I also want to do the mummy - I have ivermectin which I used on her just before we got the eggs.  I wondered if I had used enough on her to be effective though.  If I use the lice powder on the chick and the ivermectin on the back of the mummy's neck would that be ok?  Also, could I sprinkle some of the mite powder in their house.  They are on their own in their own house and little run within the bigger run with my other hens.