ok---there is really no such thing as non-flammable flannel- it is cotton which will burn just like all of your other quilting fabrics----there are flannels on the market that have a non-flamable coating on them designed to be used for pajamas---the coating actually washes away in about 20 washes- the regular quilting flannels we buy say on them (not designed for childrens sleep ware) because the government says they have to say that- we all make flannel quilts including baby quilts with these flannels-
as for the bulk- i generally press to one side & nest rows so they line up nicely
batting in flannel quilts make wonderful, fluffy/cozy/snuggly quilts i (personally) do not like flat quilts- but that is my opinion- especially where there are many seams involved- the batting pads all those seams-
but it is up to you whether or not you choose to use batting- if for a baby in a cold climate i recommend it though.
if you do use batting a walking foot is helpful but can be done without if you lengthen your stitch length- the pressure of the foot and stitch slowly.