Hi, Ziggy, welcome to The Reef Tank!
I would definitely recommend against the Tang or Copperband in such a small tank. They need lots of swimming room. Mandarins require a steady source of live food such as found in established reef systems with lots of live rock or substrate. They usually starve in new systems.
Your best bet of the ones you mention is a pair of Ocellaris or Percula Clowns. If you get two juveniles, the dominant one will develop as a female. These are hardy fish and a delightful addition to any aquarium.
Since your tank is new, I would wait at least 6 weeks before introducing any fish. This lets the system stabilize and it's much easier to deal with the expected series of algae blooms that new systems go through. This also gives you plenty of time to research and plan out your tank's occupants.
I would recommend getting a copy of
Scott Michael's
Marine Fishes Pocket Guide. This book gives you good info on care and feeding requirements of 500 fish found in the aquarium trade.
Good luck with your new aquarium!
Dick
