I use mixed fabrics all the time. I have used flannels, denims, linens, corduroys, fleece, minky, poly/cotton blends and woolens all with quilting cotton. While I do not prewash most of my quilting cottons, I do however prewash some of the above fabrics before using them in a quilt. You can always make a mixed material mini quilt or quilt block & wash to test your fabrics cohesion and reactions. I am finding that making mini quilts or test quilt block sandwiches is a great way to work through design or quilting issues.