I mix somewhat, but try to keep from using extremely thin fabric with heavy fabric. Ie. not using batiste or voile with denim or corduroy.
I've not had problems mixing homespun, batiks and regular cotton or even poly/cotton. In scrappy I've used some that I have no idea of fiber content.

"I do it all the time. I also mix washed and unwashed fabrics in the same quilt. I've never had a problem. My quilts are not works of art not are they heirloom pieces. They are meant to be used and washed and dragged around."

The only person I knew that quilted was my Grandma. She used any available material in her quilts. So...I had a wildly patterned skirt and shirt that had been given to me years ago that I did not wear. I decided to cut it into 5"squares and make the sashed disappearing 9-patch from Missouri star. The material is nylon, the sashing is cotton. It takes a little time and when I am finished I will call whatever I make "Maggie" and I will think of my Grandmother every time I use it. I remember seeing my old clothes in her quilts, such good memories of her.

When I first started quilting I mixed decorator fabrics, new cotton fabrics, and recycled clothing from the thrift store. They are all still in use and no one has complained about how they feel. I still do this sometimes with scrap quilts, but now that I have a good sized stash I don't do it so much. Sometimes I make a quilt just from the decorator fabrics that DD and I made purses from several years ago. I still use a lot of thrift store fabrics because sometimes I run across such a unique pattern or a great color I just can't resist.

The only time I have mixed weights and fibers is with a crazy quilt. As a handquilter I know how difficult it can be to stitch through different weights and blends of fabric. I don't care to use poly blends as they tend to pill with use. I will confess that I often don't prewash my fabrics but so far nothing has bled.