In most of your mentioned market caps you are mostly looking at pinksheets.
I personally like to troll these but you dont see lots of people talking about them on COBF because of a myraid of factors. (unclear financials, lack of liquidity, misaligned management incentives, risk/reward profile) There are some better boards/forums for people who actively invest in nanocap companies. (investorshub, /r/pennystocks, depending on the name you can find decent stuff on seekingalpha)
To answer a few of your questions.
My question is, what should be my cut-off on the lower end as to whether the company is too small to viably sustain itself? - IMO as low as you feel comfortable. A company can sustain itself at any size depending on who is running it and how they are running it. Here is the kicker if its on a regulated exchange such as the NASDAQ then it is subject to a much higher standard which incurs a bunch of costs. The benefit here is if they are on one of these exchanges you can feel much more confident in their numbers they are reporting. Whereas, the majority of public companies in the marketcaps your describing will be listed on the pinksheets / OTC and they wont be held the the same regulatory standards.
Do I only look at stocks with a market cap of $25m USD and upwards? - Up to you.
Is that silly, should I also be looking at $5m to $25m USD market caps? - Again up to you, There is money to be made in the lower buckets its just a slightly different game.
Is a company with a market cap of $4.7m USD for example too small to reasonably make enough money to comfortably cover its ongoing admin and listing costs for the next 10 years? - Sure, lots of successful small businesses out there. Question is why are you investing in it? is there a viable path to 10M? 20M? 30M? is it a 4.7M marketcap with 25M of had assets on the balance sheet? is there a way to as an investor to unlock said assets? Will there be a sale of the company? -- The reality of most businesses public or not in these size ranges are not to be providing returns to the shareholders but to be a business that is providing for its direct employees. Many of these companies will have heavy family ownership and the people making money are the management not the shareholders.
My advice if your going to continue down this rabbit hole. Really do your due diligence. if you're looking at pink sheet companies do some real scuttle butt. Remember that in these marketcaps its often small companies who don't have the investor in mind but are just trying to pay themselves and service their niche. So what is the value unlock your expecting? What is the timeline you expect that in?
All that to say. its fun if you're into it. Probably more effort than its worth. I've made money on several nanocap. I'm holding bags/lost on some nano cap companies.