I've made a couple of fleece peg loom rugs , it's a great way of using up all the odds and ends of rougher fleece or that scratchy one that someone gave you for free and you don't really feel like spinning. I do always wash it first, even just good soak in cold water gets rid of a heck of a lot of dirt and grot. It's best in my experience to leave a bit of the lanolin in to help hold the staples together as you tease and twist them out a bit. I did try one using very roughly spun thick singles but it looked much the same at the end so not worth the extra effort. If you leave a little lanolin in the fibre then it makes the rug a little more dirt resistant, especially to spills. I only wash the rugs after a year or so, by that time the cats have needle felted them together a bit and there's no chance of the mat coming apart.
Little fleece make great seat cushions btw, I have one for my camping chair. (No lanolin left in that one, I don't want a greasy bum!) And I made a long seat cushion for each of my garden benches out of long rag strips torn from charity shop quilt covers, they look nice and fresh.
The usual knotting technique is to knot the first three threads together at the bottom, then in pairs across the width, then the last three. That knots one thread from each adjacent pair together and stops the weft slipping off the bottom if you pack hard down, this is more important for slippy cotton weft than fleece though. Fleece grips, especially if you use a wool warp.
