As far as I know, all the fabrics are safe for chewing...some just get destroyed much easier than others!

Fleece is great because the inevitable holes won't fray like flannel or cotton, but I certainly use those as well in their accessories. My rats think jersey is the tastiest!
Poly-fill should be fine. They probably won't take any nibbles, since I can't imagine that it's very satisfying to chew on. It just might get a bit messy for you!

I like to use fleece scraps for nesting -- you can wash them in a lingerie bag all together, and they don't get lost that way.