Mine is 4mm diameter.
Pressure should not matter, you set the floe rate not pressure. My reg was 50PSI and no problems (set to 10LPM or 20SCFM.)
My clark came with the appropriate regulator. My gas supplier would have the fitting if i needed it.
When I changed to a different gas setup, I used some 1/8" vacuum line slid over the 4mm line from the clarke. From that went to some standard barb fittings.
My Clarke did aluminum welding. It was not ideal for aluminum and took a a lot of practice.