What is your favorite shape ? If you intend to tumble beads with grooves and you want to alter the material in the grooves then a triangle shape works great. For doing what you want any shape they sell would work. Remember to keep a close eye on the tumbling progress.Since your beads are of low hardness you could even run the media and grit for 15 minutes to get it mixed in well then add your soft beads. I would check them after 20 minutes to judge the progress.Watching the change in that manner is very pleasing to me.