I mix regular cottons and flannels all the time, even batiks with flannels. No reason not to. I never prewash ( precuts) and have never had a problem using them with other cottons. There are lots of factors that figure in to shrinkage in quilts, including batting, backing, and type of quilting. I mix prewashed and unwashed fabrics, flannels, regular cottons, all kinds of types. I Wash and dry my completed quilts. Ive never had one turn out ( upsetting/ bad/ disappointing)