I got a yellow watchman in my 3g pico. Some say thats still too big, but if you actually watch the fish, it stays w/in like 6 inches of one spot. It did the same thing in my 29 as well...never left one spot for the most part, so IMO its fine in there. If you were to go w/the 5, you could easily do a YWG and a pistol shrimp pair. My YWG has a randalli pistol shrimp w/him in the 3g pico and they do great together.
But if you do the 2.5g AGA tank, those are very easy to light. I think that the Coralife Mini Aqualight (2x9w pc's) fits perfectly over a 2.5g AGA tank. I'm using one of them on my 3g pico and i've got great results so far w/it.