I personally don't agree with keeping plecos in unheated goldifsh tanks. Plecos are from the Amazon, about as tropical as it gets.
Another problem with certain plecos or other sucker type fish is they can harm the goldfish. Instead of sucking off algae from the glass and hardscape, they go after the goldfish's slime coat. Guess that tastes yummier to them than algae does. But this causes serious wounds on the body of goldfish. Open sores, infection, and all the rest.