Honestly, we make the rules and we should be able to change them. That said, I wouldn't put flannel and a woven cotton together to make a top. Flannel is cotton, but the weave is much looser than the cotton fabrics that we make tops from. The 2 will shrink differently. Even if you pre-wash. The minkee fabric or fleece I have seen used on the back of a quilt that a long armer did. It created a whole new type of texture. I wouldn't do it on my home machine, because I can't 'stretch' the sides, to be flat. It would be more like a lake on the bottom if I did it!