yes, you can combine fabrics. and fleece or flannel will work fine for the back. Since flannel shrinks a lot, wash it a couple of times before you stitch it all together. Fleece won't shrink, but washing will help get out extra "fuzzies". I happen to think that flannel and fleece are both warmer than regular fabric...have you thought about using a muslin instead of batting with regular fabrics for the front and back - that might be cooler than the flannel or fleece.