I use the cheapest copy paper I can find, and don't usually have a problem with the stitches coming out, but another suggestion I have, since you have shortened the stitch length, is to perforate the paper before stitching, then go ahead and sew your fabrics on. You don't say how much you shortened your stitch length, so that might be another possibility. My machine's automatic length is 2.8, I shorten it to 1.5.