| I would say that you are good to add a couple more fish (slowly) and maybe some more coral.
What are you running for lighting, this may determine what kind of coral you should add. Also, like Matt suggested, the eel may not be a good choice, but there are many other cool fish that may be happy in there. Some hardy ones that come to mind are Wrasses, Firefish, Chromis and Gobies (some), | 
