I would not be afraid to combine prewashed with unwashed. It is rare that someone prewashes batting and it does shrink. I have used backings that are unwashed as well as sheets that have been washed multiple times. I think your question is combining washed and unwashed in the top. I've never done this but you could try it on a small scale. Not all charms are 5" square out of the package so you would need to measure before and after to know for sure. Then there's the batting, backing, seams, quilting, all affect the final size of the quilt.