Mrs. Fluffy Puffy :
I have told my mom over, and over again and she said I couldn't use that because it has too much toxic stuff in it.
If I were your mother that is what I would tell you- don't sacrifice your future health for your chickens PLEASE!
When you are 40-50 years old, you don't want to look back and see that you were inhaling toxic pesticides as a teen. I just have been through treating my about 80 chickens for mites, and let me tell you, I BREATHED in the Sevin and permethrin dust. And the Eprinex pour on got all over my skin-ugh. But I'm in my 40's. And have already had my kids.
And it has to be done over and over since they will keep getting mites from the songbirds.
You can try DE and that will have to be good enough (according to your mom), unless you are able to access some of the pour-on mite medications (like Eprinex). My Eprinex was I think about $45.
I hope you can get your chicken run done if that's what you want to do.
If I were in your shoes I think I would keep my favorites and sell the rest at auction (and I even made a thread about how horrible my auction experience was). Then I would make a manageable run and try to build a smaller coop within the chicken pen that you have (I mean fence in a section of it to make it predator proof, and put a top on it).
Presto! Smaller, predator-proof area including your roosts! And a outside small run so they can get the sunshine. The horses might knock it over again, though, right? I don't know what to do about that.
You'll find a way! If you want to keep all 100 of them, go ahead! Just go the nontoxic route and do the best you can!!!!