i mix textiles a lot. i have used challis on the back of a quilt that gave it a designer touch, i have used wool blends, silky cottons, etc. testing fabrics ahead if you do not understand the properties of them is probably best. i have been sewing many years so i am famililar with properties of many fabrics expecially the ones in my possesion.
the flannels in my stash were higher end flannels that i bought many years ago and none of the quilts i used them in had any significant shrinkage. the flannels i have purchased in recent years i preshrink because there is so much shedding and shrinkage that occurs when washed.