Crown bags along with regular cottons are fine together. You've already washed them so now go for it. You stated you have already cut your squares, I Always starch my flannel well, but before I cut so might be too late for that. Many of us use flannels and regular cottons together all the time. I love flannels and regular weave cottons together. Use a good, new, sharp needle & either starch or add a fusible stabilizer if you find they are too stretchy. My crown bag quilt wasn't too stretchy but I starched well before cutting so I don't know how it will be without the starch.