Welcome from Medicine Hat, Alberta!! I have big Koi and they need lots of room for sure, like Sissy said. I would just get a half dozen small cheap goldfish for now to so how it goes, New ponds are the hardest to stabilize, high PH and ammonia could be a potential problems in the beginning.
I have been wintering my ponds in Canada successfully for over 22 years now, and will be glad to help were I can. Have you got any pond plants at all yet? Did you find any pond plants left over from last year that show any signs of new growth? I can help with plants and so can lots of people here too.
You will want some good hardy dependable pond sedges that come back every year as they are best at photosynthesis filtration. A big hardy Sedge will clean the water better than anything else. It will take a few years to get a good size clump to live year round in the pond. Sedges are any spear shaped plant, like cattails, rushes, water iris, and grasses. Tall sedges also provide shelter for fish in winter and summer, and shade the pond without smothering it.
You can read more about my pond under garden pond photography, Ponds in Canada thread. New to computers and I only have a tablet, but I can post lots of pictures!
Here are some of today's pictures from my pond, the big water iris (yellow flag) is in the second picture and it is slowly coming back to life in Canada...
pictures of the bottom pond, (three connecting ponds) from last year, so you can see how tall my grass like sedges are .... and the last picture you see the tall Yellow flag water iris, blooming yellow from last spring.