I have had a two spot(four wheel drive) goby, and he was great. Peaceful, good sand sifter and cool to watch hover. Yasha goby with pistol shrimp is very interesting to watch, however they are shy. I've had a yellow gumdrop goby, who was fine alone. Had two, and one picked on the other. Firefish are cool too, but seal the back, because i've found him on the filter tray. Emerald Crab, Porcelain Crab, Boxer Crab pretty cool too(off subject). I have a Cherub Pygmy Angel, but he's a little outside your parameters. Have fun.