You're not really going to have enough light for any coral. You're not looking at the wpg, but how much light your actually putting out. 8 watts is still going to be 8 watts no matter how you look at it.
You might want to look into one of the small fixtures for nanos. I think there's one that incorporates 1 13w 10,000K bulb and 1 13w actinic bulb. I used that setup for zoas, mushrooms, different polyps, xenia, and a small open brain in a 2.5g.