Welcome to the group!
Since you typed it twice, it sounds like the 100 gallon pond part isn't a typo? As someone who is also newer to ponding, I too was very excited about getting fish for my pond. But you also want your fish to be healthy and happy, right? Each of your koi can reach the size of a house cat when fully grown. Even the folks over at koiphen who keep dedicated koi ponds with massive amounts of filtration to allow them to overstock don't recommend less than 800-1000 gallons for your first koi fish, and then obviously even larger for any additional koi.
Having a smaller pond myself (300 gal main pond plus 30 gallon area of bog filtration), I've learned that a couple of the big concerns with our small ponds are temperature and overstocking.
Small ponds are by their nature usually very shallow, and therefore more susceptible to large temperature swings because of this. Especially since there are no plants yet you'll want to have a thermometer to monitor what's happening with your water temperature. You likely may need to use a shade cloth in the peak of the summer, and if you live in an area that freezes in the winter you'll likely need to bring your fish inside in the winter months (shallow ponds are more likely to freeze over completely) so keep in mind you might need an indoor setup also or at minimum a pond heater.
Overstocking is a big issue because for someone who is new to ponds (with or without prior aquarium experience) the 100 gals can seem big at first, but it's actually not compared to what the fish actually need. What you can put in there will depend on both the filter and pump you are using, and the adult size of the fish. For 100 gallons, if you want lots of fish I would either look at much smaller fish that school and shoal (I have danios and white cloud mountain minnows, it can be fun to watch) or just a few standard gold fish (common, comet, sarasa or shubunkin). Slim-bodied goldfish are quite beautiful and actually get extremely large themselves (12-18" per fish depending on the type), so try to go easy on how many you put in, maybe start with 2? Especially if there's only a couple fish in there, babies are more likely to survive, I would think so that would also need to be monitored. Or fan-tailed goldfish by themselves would also work (they are slow swimmers that don't compete well for food with the faster fish) and they are a bit smaller so you could get a couple more. Plants will help maintain your water quality, shade the water, and give the fish a place to feel safe which is especially important for the smaller species.
Good luck!