The last time I tried using cotton/poly fabric I was making doll clothes. The problem I found with it is that it was hard to press in the same way that all cotton presses. I like a nice sharp crease on my blocks especially. Didn't like the way it turned out for doll clothes and I think it would be the same in a quilt block. You might do a test block and see what you think. If you have enough to make an entire quilt with it, it might be better than mixing. Maybe a utility type quilt to keep in the car or for a picnic.