It all depends on the look (and feel) you're going for. I've mixed corduroy, cotton, and flannel in rag quilts, knowing that the result would be interesting textures. If you want the peachskin to be indistinguishable from the cotton, then no, they shouldn't be mixed. But if you want to use a bit here and there because it is just the right color or texture, then go ahead. Be aware that the peachskin will not behave like the cotton when it is washed, and it won't have the same feel while you're sewing with it. It may be more slippery. Since it's polyester it won't shrink and will probably retain its color even as the cotton fades a bit. Maybe you could experiment a bit with it before committing to using it in a quilt.