Gas Types for 33g Cylinders

I'd check with several manufacturers and choose the best one.

Apparently not, but these may be other than 33gms each, or with bayonet, not threaded necks

If I understand your post right, you want to have a 33g size gas cylinder, that is intended for air pistols and rifles, but use a different gas.

Understand that these cylinders have CO2 in a liquid form, so they can provide propellant for many firings. CO2 can exist at room temperature in liquid form, and it's state is what is called saturated liquid/gas. Any other gas that would have sufficient pressure, would exist in gas form only, as nitrogen, oxygen, argon, etc. are beyond their critical temperature (unless chilled to a very cold temp). So you would have limited firings of an air gun using other gases.

Am I understanding this correctly?

Those cylinders are used for other gases. N20 and Helium being two others. The N20 are used in a whip cream dispenser.

There are laws regulating what gases can be place in them which vary from country to country.

http://www.ntg.co.jp/english/products/