Harry, Harry, Harry - foregoing your 15 mm!?
You can also look at Irregular Miniatures, they have plains wars Indians, slouch hat figures from ACW, and hunt around the Colonial range for a few things - like a gatling gun (need to replace the crew likely).
Both Irregular and Pendraken have animals you can use. I don't think I'd mix the people from Pendraken and Irregular, maybe in separate units, but the animals and scenics from Irregular might work well.
Do keep in mind that Pendraken's figures tend on the smaller side. I have their Vietnam stuff and had I known how small they were I would not have bought them but will use them, especially with all the heavy equipment they make.
If you are still chasing a Pony Wars size collection, keep in mind you do not need anywhere near as many mounted Indians as they used, just recycle. I went seriously overboard with my 15 mm Old West/Pony Wars but you don't have to, whether in 15 mm or 10 mm.
My biggest recommendation, is stick with 15 mm and reinvigorate your mojo somehow.