That is too many fish, i would get rid of the damsel at least (they are totally evil fish anyways and have been know to bully the other fish to death). You might be able to add the 5th fish, but if you are getting algae or nitrates before adding the 5th then you know you are at your stocking limit already. But i would say 5 fish is the maximum you can have in there.
And that is great skimmer. Just make sure you get one that is rate for twice your system volume.
I am totally going to sound hippy dippy here, but listen to your tank. It will tell you when you are at your stocking limits.