Interesting start...
I have Qt’d my koi in the past in a 300 gallon Rubbermaid tub for 4-6 weeks. Pretty much just observed them to make sure the showed no signs of illness then released them into the pond, didn’t treat them unless they showed signs of illness. Recently in the past couple years I bought koi online from a vendor that qt’s all their fish before offering them for sale.
Appropriate Stocking levels are hard to say, as everyone’s set up is different, but will say everyone eventually becomes overstocked, unless you put some type of fish in there to eat the fry. Most say about a 1000 gallons each Koi, but I was never crazy about the cookie cutter stocking level thing. For the above mentioned reason.You can get away with more fish in my opinion, using an appropriately sized bog/wetland filter and heavily planting the pond itself, as well as proper circulation.
My pond for instance is roughly 10,000 gallons, and with out a doubt is overstocked with various species of fish (too many to count) and 7 turtles, but I recently enlarged the bog and the pond is heavily planted, so it stays in balance. (It’s going on 11 years old and hasn’t crashed on me yet)...It also matters what plants you use in the bog and pond ie quick growers whether it be through root spread or runners For nutrient uptake. Personally , I prefer aggressive quick growing plants that can be weeded out (nutrient export)easily without having to rip out massive roots.