Well,
As the only one that has responded to this that actually has a pleco, koi, and a pond, I can say that the pleco will be happier than a pig in you know what in your dads pond, providing the water is above 65 degrees. But you will have to over-winter him indoors for sure.
Pleco sucking the slime coat off of a koi? I kinda doubt it, never been an issue with my koi, it is usually the koi that can out-compete the pleco for food.
Will it produce more poo than it consumes? Well, I am not a biologist, but I did stay at a holiday inn last night and this seems like a physical impossibility. Besides, ahem, Koi are monster poopers themselves, and I would suspect the pleco would give you the same addition to your bio-load as a simular sized koi.