I have fly fished Raro and Aitutaki a few times and will going back to Raro again in August. I have found you will catch plenty of fish of all kinds (but mostly small) using a smallish clouser and a twitchy retrieve cast close to just about any patch of coral, the main chanels leading in and out of the lagoon, like the one at Muri and the one right in front of Trader Jacks can give up the odd bigger fish using bigger flys and poppers and should give you a shot at some good Bluefin Trevally.
You will see lots of very large mullet cruising around in the lagoon, they are very hard to hook, but if you do manage to hook one they fight very very well! I used a very small shrimp pattern to hook the only one I managed to fool, but the same fly sent some other mullet fleeing in panic!
Some of the reef fish you are likely to hook are Picasso fish, Yellow Tail Goat Fish, Flounder, Cornet fish, Box fish, Bluefin Trevaly, Emperor Grouper, Flame tail snapper and many many more.
All the best.
Mike