I avoid mixing polyester fabric with cottons....they don't seem to work well together as far as piecing and quilting are concerned. I learned the hard way when I FMQ'ed a quilt made by a friend's mother. At first glance, it seemed alright until it came time to load it on the frame. It would not lay flat and there were wrinkles beyond control. I simply quiltied the heck out of the wrinkles to the point the folds from the wrinkles were not seen anymore! I explained to the friend who was very understanding. As for art quilts, I don't see any problems with combining different fabrics.