I use fleece and minky on many baby quilts. I still use a thin cotton batting, but you don't have to. I have had no problems with minky: I just use lots of pins all over, but then you have to do that even with cotton backing. Maybe the cotton batting helps to stabilise it, I don't know, but I've done both straight and free motion quilting and it's been fine, making a cosy and soft quilt.