I buy walnut media at the pet stores, they sell it as lizard bedding. I also decap before tumbling. The walnut media pours easily from bottleneck cases, but the smallest thing I tumble is .223, so I can't address anything smaller than that.
I don't add any polish to my media. I don't care if they're shiny, Clean is good enough for me. I use the same media to make sandbags for range shooting. It's a whole lot lighter than sand and seems to hold up well.
Which reminds me, it's time to cut the legs off some more bluejeans.