No fleece at all. Save your money. Yes straw can have mites. That's part of why I use hay, it also gives them something to chew on.
I would be concerned that they will pee on it and in cold temps could freeze. I had fleece for my Guinea pigs indoors and cleaning it was awful. I would spray it out with the hose then wash it. Hair everywhere and it smelled like Guinea pigs even when clean.