I actually don't see how "poo-eating" fish would be useful. The fish would eat the waste, and in turn it would pass waste. Either way the waste would still be in the tank.
Worms do a good job of breaking down waste, but they don't eliminate it. I think they do make it easier for plants to process though, and they also help keep the substrate from forming anaerobic pockets. I'm not sure what kind of worm it is that I have, but I have some in my 29 gal. I thought they were bloodworms, but now I'm thinking they are tubifex. I didn't intentionally put them in my tank, but they don't cause any problems and actually I think they are a benefit. If I knew what they were exactly I would get some for my other tank as well.