That 20/10 thing isn't for long-bodied goldfish or ponds. It's for round-bodied goldfish kept in aquariums with 10X filtration and weekly PWC's. Even then, it does not work for long term success. An 8" round bodied goldfish will not do very well with only 10G per goldfish... even with 10X filtration and weekly PWC's. They are equal in body mass to hundreds of 1" goldfish.
Ponds usually get 1-2X filtration, have a lot more waste issues from fallen leaves, etc., and long-bodied goldfish need 50G each as a minimum as adults. They grow to 12"++ and up to 24" depending on the species and genetics. They are much more likely to breed in a pond so you would find yourself quickly overstocked with stunting and health issues.
22" in the ground is a pretty good level and should benefit dramatically from geothermal heating/coolling in the winter/summer. Still read up on overwintering and prepping the pond for the winter time as far as cleaning, when to stop feeding, when to turn off the pump/fountain, adding a surface bubbler to keep the ice from freezing over completely, etc.
I think your plans for 3 is a good plan to start off .... and if you can fashion some kind of screen to prevent the minnows from getting washed down the waterfall, the 35G section would work for them or for growing out baby goldfish next spring. It's also a good place to have lots of plants that will aid the ecology of the pond without being eaten by the goldfish. I'm not sure it wouldn't be more likely to freeze up over winter so check on that aspect. You wouldn't want ice to break it open.
Depending on the water temps, it's not always good to run the filter year round as it messes with the thermoclines of the pond by circulating the water. Some people use a smaller pump at the surface to keep running water slowly going through the filter to keep the filter as alive as possible without circulating the warmer water from the bottom of the pond. You probably won't run your waterfall year round for this reason also.
I'm glad you can't wait to start digging! Post back at the end of that first day and let us know how you feel about digging!