I would recommend a royal dottyback or royal gramma. The fomer is a littler feister but will eat bristleworms if you ever get any. They look pretty much the same, but the Gramma is more beautiful. Dottybacks have lots of personality and the Royal Dottyback is one of the most odd fish of the dottyback family. I have one and he's great! Onlly picks on the stupid damsel I have that hasn't died and it's been 4 or 5 years now! One thing though, if you have a firefish, a dottyback may not be idea. Go for the gramma.
How is your live rock content? You should ensure you have live rock built up to about half the height of your tank to provide sufficient hiding places and refuge for all your new fish.
Yellow Clown Gobies are a tiny addition that will thrive in most aquariums. Just make sure you have branched live rock or some synthetic coral with large branches. They'll love it. I've read some dottybacks shouldn't be kept with gobies, but my royal dottyback, the most aggressive of the group doesn't bother my clown goby.