Though too small for a tang, at least long term, there are some dwarf angels that might work.
Cherub angels, stay small and have great personalities. They also like to eat algae. If you'd like something flashier a flame back angel would be similar, though they can be a little more aggressive.
Flasher and some of the smaller fairy wrasses would add some nice color and motion to the tank.
Most blennies and gobies are good choices.
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!)
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)
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).