+1 for the pistol goby pairs, symbiotics are really cool, sounds perfect for your size tank.
I also had an Emerald Crab when i first set up my 20 gallon, he ate all the hair algae in my tank by his lonely, and then proceeded to quadruple in size. That tank was his. He would stand on the top of his rock and wave his arms around like a boss. Caught him one morning with only my little black Clownfishes fin left. He never managed to kill any corals or inverts though. Sent him to a 2.5 gallon beta tank jail with some rock rubble, but he didnt last long in my roommates room. I still miss Kramer the murder crab though, he had tons of personality.
But how about a couple of Harlequin shrimp? really cool looking and they eat starfish almost exclusively.
http://piratefx.hubpages.com/hub/Harlequin-Shrimp
If your going for zoanthids or mushrooms beware the sexy shrimp, i have a couple in my tank and they have pretty much destroyed my only zoa frag.