Can/Should I add another?

[ndwatso]

So I though I had finished off my 55 Gallon Tank with my 3 fish (scotter, percula,bangaii) however I decided to hook up my 60 gallon sump to my 55 gallon system since I will not be finishing my 200plus build for a few years(still need to put up the wall its going in).
With the added water volume I think it should be safe to add one more fish. The fish in my current set up (percula and Bangaii ) tend to stay on one side of the tank and when one moves the other does as well. With those 2 only using half the tank I room should not be an issue.

This is a FOWLR tank.

So what to get with these 3? Here is a list of those that I like in order:
Fuzzy Dawrf
Harlequin Basslet (or maybe a swiss)
Longnose Hawkfish
Orange Spotted Goby

[BIG_KAHUNA]

Longnose hawfish have awesome personalities. Freekin hilarious!

[ndwatso]

My question about the longnose is will it get along with the scotter?

[Dragon moray ki]

You could do a small Niger mine is very freindly and grows very slowly

[akdylpcikles]

You could do a small Niger mine is very freindly and grows very slowly

That's a very bad idea. Considering the Niger trigger can grow up to 18 inches long

[AlexS95]

+1...

[Matthew91]

Fox face perhaps would be a nice addition

[Monkeyfish]

Foxface will outgrow that tank.

I think any of your original list would do fine, though the dwarf lion may eventually eat the perc.