its ok to do what ever you want- there are no (real quilt police-only a few people who think they are)
the only thing to take into consideration when deciding on mixing fibers is laundry-care...if you can wash it all together- you can put it all together-
when i use fleece for backings i make sure i pre-washed the cotton fabrics-
i also include care instructions on my label- when using fleece you need to make sure the quilt is washed in cold water-
it's the same as using minkee- which is a polyester fleece.
i've combined many many different fibers in the same quilts with great success- i love using wools- and silks are just fabulous!
my granddaughter's always want minkee or arctic fleece.