I have also used a "stretch" straight stitch to sew denim seams--(goes 2 forward then one back). As to weight, the bigger the patches, the lighter the quilt as there are less seam allowances using a bigger patch--say 8-9 inch squares verses using 5 or 6 inches.
Rag quilts sound great.
Just finished a baby quilt for new grandbaby. Used 8 inch squares that I serged seams on. Think I had 36 squares (6 x 6). Utility stiched diagonally across each square, lightweight batting and flannel backing with bound edges. (Probably more work than what you want to do)
Great project--they wear like iron--good luck