Depends on how cold it gets. The 'Common Pleco' is actually any of several species, and there is not a good way of knowing which you have. Some are a lot alike.
Certain Plecos survive in Florida in the wild.
Farther north, colder water they may not. I would plan on heating the pond or bringing the pleco in over the winter.
Koi stocking levels depends on the size of the fish and the equipment (including plants) in the system. Figure it at the hardest time of the year. Usually this is a choice of heat = less oxygen or cold = dormant bacteria. Whenever YOUR fish are stressed, that is the most critical time.
Monitor the pond conditions. If the parameters are getting out of hand and you cannot deal with them, your pond is overstocked.
Here are some numbers from a UK site, so the gallons are about 20% bigger than American gallons.
recommend following gallons per Koi rates based on their size:
Small Koi (2" - 8"): 100 - 150 gallons
5cm - 20cm
Medium Koi(8" - 14"): 250 - 300 gallons
20cm - 35.5cm
Large Koi(14" - 24"): 400 - 500 gallons
35cm - 61cm
Jumbo Koi(24" - 36"): 750 - 900 gallons
61cm - 91cm
I think I would go with more fish per gallon in the Small (2" -8") grouping if all the fish were closer to the 2" end of the scale, but what would I do with half a dozen Koi as they grow? (and they can grow fast when they start out that small!)