So agree.
Common Pleco's grow to about 12" long, When they are babies (which most people see in the LFS) they are herbivores, then when they reach the 4-6" mark they become more hungry and going to the omnivores stage and unfortunately the goldfish's slim coat is a great source of food. Now if a goldfish is a slow mover or is slightly sick the Pleco doesn't know better and will try to attack the fish out of fish behavior.
Pleco's also need wood and lots of hiding places, they are a nite time animal so you don't see them much during the day time, so most people don't know what there pleco is really doing.
I have been one of those people that had one with my goldfish and he dragged down a 6" goldfish my pleco was only 4" long so it doesn't matter about the size of the pleco, more the temper of the pleco