I've been thinking of nabbing one of those seahorses off Colby too, but ya the Tang would have to go, and that would be sad!
Too bad you already have the ponies though, since I would seriously let you borrow "Skipper". He's easy to catch (well...in my tank anyway!). Take a peek at Hector's Gobies. I think they mainly go for hair algae though. They wouldn't be competitive with the horses...they won't usually eat prepared foods.
Are your ponies eating yet? What are they eating? PE Mysis?