Plecos go not eat poop. And all plecos are not vegetarians. I have been breeding Hypancistrus plecos for a couple of decades and they are primarily meat eaters. More accurately they are omnivores who love meat. I do feed a bit of veggie matter as well.
I just did a quick search (15 mins.) on Google Scholar (
https://scholar.google.com/ ) looking for information on fish which eat feces. Pretty much there was nothing in the FW world. But it turns out that fish poop does feed corals. In fw it becomes fertilizer for aquatic plants. In a tank poop will break down and then settle into the substrate. Decaying organic matter makes ammonia which is plant food. If one has no plants, then the bacteria/Archaea get the ammonia.
A couple of species of sw fish are thought to eat feces, but this is not certain if I read right. It is though that they are after the microrganisms in the feces. This is similar to the wood eating plecos (Xylivores) in FW.
If anybody can find research papers showing that any species of fw fish eats poop, please post the info or link here.