I have mixed cottons and cotton/poly blends in scrap quilts with no immediate problem. On one quilt for my dog one piece of poly/cotton blend did not hold up as well as the rest of the quilt. I solved that problem by adding an applique flower covering that block. That has happened only once but I have found some old utilitarian quilts that failed in the same way. Generally there should be no problems. I look forward to seeing your finished quilt.