I have used poly/cotton blends, and have not noticed a problem with them. I believe you can mix them, but you need to make sure that your cotton ones are preshrunk, as they do not shrink the same. I just automatically prewash everything, because I never know if I am going to use if for a quilt or an outfit for one of my granddaughters. Sometimes, I will take the leftovers from little dresses and stuff, so I just preshrink everything.