A clarification, since this thread is about regulators too...
You can set a regulator with a gauge with either gas. That static pressure will work on either gas as long as you are not "shooting" the CO2 into the regulator.
Also, lower pressure (200-300 psi) CO2 behaves closer to HPA, and you can tune those post regulation parts pretty close together. However, flow through all of the fittings, regulator, and valve interact, so if you are looking for the best settings, you need the same gas.