To remove wrinkles from fleece (I never iron the stuff -- too afraid it will melt) I just put it in the dryer with a very wet towel. It only takes a few minutes. Then I hang the towel to dry it.
If you have something with a sprayer so you can mist you could dampen them some. Do you know if your iron has a place to put water in it to make steam? Distilled water is best but I have used tap water when I didn't have any distilled water. If it does have a place to add water tip the water out when you are done ironing.
Water should help on the muslin. I also always spray with Mary Ellen's starch available at JoAnes, Hancock Fabrics. It's nice and does NOT leave a residue. I don't like fleece so can't help you on that one.I would imagine it could burn though.Be caref