Most pleco's are tropical, common plecos are tropical going on sub-tropical but there are no true coldwater plecos as far as i am aware. Because plecos are bottom dwelling fish, they need the subtrate in their tanks kept very clean and because both plecos and goldfish are big waste producers, you need to have very strong filtration and be very vigilent about keeping the bottom of the tank clean. In a pond this would be a problem, as most goldfish ponds need to be at least 3ft deep and have many gallons, its not very practical to clean the pond of a pond on a regular basis like you would with an aquarium.
The only way you could get away with it is if you heated the pond and over-filtered the pond, however heating ponds is rather expensive and so is buying decent pond filtration, however if this was not a problem then you could keep a pleco such as a common pleco in a pond. The only problem with keeping common pleco's with goldfish is that they have been known to try and suck the slime coat off goldfish like a Chinese Algae Eater, however this on the whole is quite unusual behaviour in common pleco's, it would probably be more likely to happen if you kept fancy goldfish with a common pleco (however fancy goldfish would also be most suited to sub-tropical temps).