Air fittings and measuring

I'm trying to figure out this whole air fittings thing. I know the difference between male and female flare fittings (eg MFL) and pipe fittings (NPT, MPT) but I'm having trouble with sizing.

If I go to buy say a short section of pipe to attach to a CO2 line, what size do I use?

I see 1/4" listed all the time, but I have no idea what that references, and I'd like to know. Ditto with thread pitch. And the flare angle; is it constant for the size of the fitting?

It seems that the conventions are different in the brewing world vs the welding world (in welding I see tanks measured in cu ft, in brewing it's lbs).

Also, I have tons of fittings laying around for air hoses for air tools. They all are 1/4", both flare and pipe. I also have quick disconnects for 1/4" both auto and industrial (one is slightly longer than the other IIRC). I'd like to use some of these with my CO2 system for the keezer, and I don't wanna go buy stuff only to have it prove incompatible. And I need to use the CO2 tank with other stuff.

You can use the air line connectors for your LOW pressure sides most here do not because of the cost and the fact that they are big and bulky. The high side of your regulator should be rated for at LEAST 1200psi for CO2 and 4000psi for Nitrogen. The air line Quick Disconnects should be 1/4 npt and thread into the low pressure ports without a problem, I use a crap load of these at work and they do leak which when running off a compressor that is constantly putting out 100+psi is not a problem BUT when you have a limited gas supply ANY leak is a tank killer. Bottom line I have at least 2 dozen of these fitting and I would never use them. I am currently changing all my low pressure gas lines over to John Guest fittings (aka. push-lock) as I like the sealing quality and the ease of change out. Then again this is just my opinion you my have better luck.