I agree with the others about rules. Now about mixing fabrics. You can put whatever you like together. Just note that on a rag quilt the finished blocks will be cut to rag out. Different types of fabric will not rag the same. If this look appeals to you, go for it. But definitely prewash the flannel. It will shrink more than the other fabs. Just have fun with the process. Your kids will love it however it turns out. My husband thinks all my quilts should be rag quilts. They are great.