Keep in mind that even though your tank is "cycled" per your measurements now, once you add fish the bioload goes up and there will be some fluctuations. I presume you used something to create an initial "bioload" during this cycling period. How closely you created and maintained an actual load compared to the load from the fish you're adding will determine how much of a fluctuation or disruption to the parameters of the "cycled tank" you have now. Typically it's hard to go from 0 to 60. Depending on how many you intend to stock in the end, you might consider doing so in stages.
That said, while it can be made a science, it doesn't necessarily need to be. Once you have some experience the measuring of parameters isn't really necessary IMO, plus once you have a tank or two going, it's easy to easily kick-start additional tanks simply be swapping some filters around.
With respect to the hospital tank, when you go with a HOB or a sponge filter, if you have it running on your main tank for a while once you've got fish, setting up the hospital tank is as simple as filling it up with similar temperature water, putting a heater in it, and moving the filter over.
I do recall you had a beautiful background created, so if running the hospital filter on the main tank looks bad, instead you can just swap over some of the biomedia to a HOB and it's as simple as the previous paragraph. The bacteria that break down ammonia and nitrites adhere to a surface, thus biomedia is typically high in surface area and when you move it around between filters, you're moving part of the bacteria colony with it. Once I have one fishless cycled tank to start, I never actually cycle another one in that manner.
