There is enormous debate and mighty few facts on fillers. If you are gonna use them, you might as well do so in a Handi with its cheaply replaceable barrels. You don't want to use much. Never more than a grain and you want that fluffed up. Cotton is heavier and less fluffy than dacron. I've tried it, but went back to dacron or polyester.
To use fiber filler, pull a strand off your supply and weigh it. A little practice will make you pretty good at knowing when you have half a grain, which is what I mostly use. I catch the end of the strand with a small screwdriver and push the end down to the powder. This draws the rest of the strand in after it. You don't want a wad on top off your powder, just a loose net filling the airspace. I've gotten away with it for several thousand rounds.
Wads are a different matter. The risk is entirely too high in my mind. If you have anything solid on top of the powder with some running room, the bullet can act just like a bore obstruction and you'll get ringing, if not worse.
I do sometimes use grease cookies. These go on the base of the bullet and I would not try them in a bottlenecked case. I saturate a sheet of felt in melted bullet lube and allow to harden. Then I cut out disks with an arch punch. Just seat them on the case mouth, place a bullet on top, and seat the bullet normally. I've seen fibers of the felt get seated along the sides of the bullet so I'm pretty sure the grease cooky will stay in place. These is a way of getting in extra lube and it seems to have a little of a GC effect. Doesn't do much to reduce case volume, though.