I was reading about twin spots, and the success rate for them is really slim. I see a lot of them in pairs at the stores, so they might be cool. They're kinda small, so with just one might get lost.
There's also those cool gobies that are mostly white, with a blue streak and yellow around their eyes... name escapes me. I see them gulping up sand all the time.
Have you seen those cool fingered dragonets?
http://www.liveaquaria.com/product/prod_Display.cfm?pCatId=1130&cc=1
Liveaquaria picture doesn't do it justice! But I've seen one in person, and WOW. Kinda cool like a scorpionfish, some have neat florescents in the fins, get larger than most gobies, and when they are extended with their fins, they're HUGE looking.