There is nothing 'wrong' with using cotton/poly. Or using denim, linen, satin, wool, fur, lace, anything you want in a quilt. It is your quilt and you can use whatever you want. If you are making an heirloom quilt or a quilt for a competition, then there are rules to follow. I believe that one of the reasons people used to say that the only true quilts were made from all cotton was that the antique quilts were usually made from all cotton - but, they didn't have cotton/poly fabrics! If using mixed fabrics, I would suggest washing everything first because they may have various shrink properties and be aware that your quilting may need to handle assorted thicknesses, but, whatever you want to do - GO FOR IT!