Here are some smaller, peaceful species which would go well with your current inhabitants, and which due to their size can make the tank seem bigger than it is:
Pink Streak ("Cryptic") Wrasse or Tanaka Pygmy Wrasse or Yellow Banded Possum Wrasse
(Be a little leary of sixline wrasses, as they can decide to own a part of the tank)
Red Headed Goby
Green Banded Goby
Neon Yellow or Blue Line Goby
Red Striped ("Trimma") Goby (love this guy!)
Royal Gramma
or Firefish: Purple or Red, take your pick
or Orchid Dottyback (Don't confuse this with purple dotty backs which can be real ***'s)
White spotted Pygmy Filefish (different)
Yellow Headed Jawfish (If you have a tight cover and several inches of medium grain substrate)
Green Chromis
Either Pajama or Bengai Cardinalfish
If you are going for slightly larger fish, then I second the long nose hawkfish (or Flame/Scarlet Hawk, love the coloration).