you should not wash an unquilted-unfinished top before sandwiching, quilting and binding...it will fray- be quite a mess
you can use what ever batting you want- some people choose to not use batting when using fleece for the back-some people do like batting- what ever you choose.
i've never seen nylon velour...i've used bubble minkie- which is a polyester fleece...is that what you mean???
minkie quilts up beautifully & makes very snuggly soft cozy quilts (my granddaughter's call them the 'get-naked' quilts- they love wrapping in them right after their baths.)