Yes you can use for mixed or st CO2 or Ar.
What's the problem?

Regards,

Gong

the deciding factor as to what you can use it for, is the nut and nipple on the flowmeter. a CGA 580 is designed for inert gas (argon, argon/co2, argon/02, helium, etc.)
a CGA 340 is designed for CO2. If the nut has external threads, it is most likely a 580. (could be a 590 for high pressure cylinders) if it has internal threads with a nylon washer it is most likely for CO2. (If it has internal threads and notches on the outside denoting left hand threads, it is probalby for a Hydrogen mix)
there are obviously more examples than this, but these are the most common.

The gasses have slightly different viscosities so the little ball will not go as high for one as another. If this was a critical application you might have to recalibrate. In general the difference won't be enough to matter.
Bill