Compression fittings are kinda finicky with plastic tubing. You usually have to use a brass insert and that restricts the flow. They are very prone to leaks if tightened too little or too much. I prefer the push to connect type that seal with an O-ring. They are dead easy, can be disconnected and reconnected over and over and flat don't leak. Here is an example for 1/8" tube to 1/8" NPT. I prefer the brass ones. There are also plastic, and now there are half and half versions where the tube portion is plastic while the pipe thread is brass.
https://www.mcmaster.com/#51025k171
Here is an all brass.
And here is a half and half version.
(Still no working inline photos on the forum software? Come on people)
Some of the fittings you linked were for 1/4 OD tube.
Pipe is measured by nominal inside diameter, while tubing is measured by actual outside diameter.
The real trick is to accurately measure and identify what threads you have. There are so many variations that I would never try to guess exactly what you have without seeing it in front of me. Good calipers and a pitch gauge will usually do the job. Most US threads are 60 degree while British (China) are 55 degrees. Measure that and you narrow things down a lot. Typically you will probably convert from what you have to male or female NPT either 1/8 or 1/4, then to the tube fittings. NPT seals on the threads while CGA and a lot of other types seal with a spherical or tapered seat and not on the threads. So while some fittings might screw together just fine, they will not seal 100%. Since you are just starting out, there is no problem just using what was supplied until you are sure of what fittings you need to make it all work. I have one flowmeter that I was never able to ID what crazy fitting was on it and I ultimately took that barb fitting and brazed it to a brass bar so I could machine it to use standard fittings.
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