As far as I know not much eats the poop, just the algae etc. I could be wrong but I was under the impression that vac gravel was the way to get rid of fish poop. There are different types of plecos that don't poop as much. I have a gold nugget pleco and compared to a common pleco he doesn't produce much waste at all.
You are not going to find a fish that will deal with poo for you. If you have algae issues and are fond of cats, plecos are a great addition, but they will add to your waste management duties rather than alleviate them.
If you are looking for fish to eat the food that falls to the bottom uneaten (this is a valuable job - my mid-water feeders let a lot go by them) then you can't beat cory cats. They add significantly to the bioload as well and gravel vacs are just as important. You are just vac'ing up poo instead of uneaten food, lol.
nothing eats poo.. it is decomposed by bacteria.. well actual feces is already 80% bacteria... anyway thats why gravel vacs are a must. However if you have a platned tank then the gravel vacs can be done less because the poo decomposes and is.. viola...fertilizer. Of course i still have lots of gravel showing in my tank so i vac aruond the plants..but im hoping the java fern/moss will soon completely cover the bottom and then i will just vaccum the tops of the plants!!! LOL