seriously? fish scaring each other to death? absolute nonsense.
plecos, esp, common plecos, can certainly *live* if you never feed them and force them to eat the crap of other fish, but here at Aquaria Central we try to give advice that will help you allow your fish to thrive, not just survive. The poop of other fish is never a recommended dietary item.
you've bred common plecos? and you didn't even feed them? wow. what size tank? what kind of substrate? how big were these fish? links? pics? unless you have a different definition of common pleco, i sincerely doubt it.
i doubt you have the experience you say you do - the question the OP asked very much depends upon what kind of fish s/he has. Ottos need algae, and many don't like to eat waste food. Plecos, on the other hand, might eat algae, or meat, or wood. Without knowing what kind of fish it's quite irresponsible to say that they all eat the same thing and you don't even have to feed them. Anyone who has kept "dozens of plecos" should know that they don't all have the same dietary requirements. If a fish is starving, it will eventually eat anything it can get, including algae that is normally not on the menu, poop, and other fish.
As far as plecos hiding all the time - again, not true. My boyfriend has three common plecos, along with several other kinds, and the commons are always out and about, get handfed, and come up to the surface for treats and other interaction. I can handfeed my bristlenoses, and all of my plecos come out when they know it is almost feeding time. Only plecos that feel threatened will hide constantly. Healthy ones will feel secure enough to leave their caves during the day.
But I guess if your fish are starved of food and attention, expected to keep their own tank clean without any help from you, and stuck with inappropriate tankmates, you could come to all those conclusions. . . .