I would like to fit Car Tyre Pressure Monitor Valve Stem Cap Sensor Indicators to my Skoda Citigo, where the cold tyre pressure is supposed to be 26 p.s.i. There is a wide range of these available, but none that I can find that are specific to this pressure. How important is it to be this specific? If it's not, then what are the criteria should I use when choosing a set?
You can go for the cheap option by searching for 26psi valve indicator caps which will allow you to visually inspect each tyre without having to get a pressure monitor to check them.
Some aftermarket tyre pressure monitoring tools (such as this one) will let you monitor whether the tyre is correctly inflated by measuring the air pressure in each tyre. These are better than the ones that just identify a softer/harder tyre by using the turning moment.
As long as they are within 1-2psi of each other that's fine in my experience, however these tools aren't much use in the event of a blow out. I've not really ever needed them in cars fitted with normal tyres but those fitted with run flats or with "gunk" to stop punctures its useful as the tyre may not look deformed due to low pressure but will affect the handling characteristics of the car.