I would go with 1/2" ID line. You can thank me later. LOL Swap out all the line for 1/2"-0.6" ID and your compressor will thank you.
Ok, understood - thank you!
Any other thoughts - for example, about the diffuser's inflow range?
Well, I think you mis understood how the disk type diffusers work. By reading the other thread, I assume you think that medium to coarse bubbles will move more water than fine bubbles. The opposite is true, fine bubbles move more water to the surface, which exposes more of it to O2, therefore more O2 is added to the water.
Since it's a DIY system, you have to do the homework. Talk to the diffuser mfg and see if you can get any specs as to how many GPM the diffuser will bring to the surface at "X" depth and with "X" CFM of air. That way it will allow you to properly size the compressor by looking at the pressure/volume curve.
"HIBLOW specs say this puts out 60lpm and 2.13 PSI." What water depth will the diffuser be placed at? Do you have a pressure gauge on the air line near the compressor? If so, what is the running PSI? If you don't, you really need one because without it you are just guessing.
Going with 2 diffusers, you will have to run 2 individual air lines, each with a ball valve to control flow rate. That ball valve will be used to balance air flow rate to each diffuser.
