But here is a question, what should I know about my pressure regulator and cylinder before choosing the metering valve. For example in one of the metering valves it says "Swagelok® Tube Fitting" and in another metering valve it says "Male NPT".
Since you are using the British standard, you obviously cannot use NPT (unless you plan to convert back and forth).
The Swagelok tube fittings are proprietary fittings, and you will just need to go to (airline) tubing. This could be a workaround if you cannot/do not want to use NPT parts.