To be honest, you can use just about any type of (safe) gas to serve coffee on draft, however over time, different gasses may start to have different effects on your coffee.
Many people think that only Nitro Coffee is served using nitrogen, however we suggest using nitrogen for any type of coffee that is being served on draft. The main difference between flat coffee and nitro coffee is that nitro coffee has been infused with nitrogen and it is poured through a stout faucet which causes the nitrogen gas that is in the liquid to become agitated and create micro-bubbles - this is what we see as the cascading effect in the glass.
So if you have the urge to use CO2 or any other gas, try to stay away from those other options as they can have an ill effect on the taste and flavor of the coffee. CO2 will begin to carbonate the coffee over time and will give it a bitter/sour taste. This can be offset with sugar, but why add sugar when you can just use nitrogen and get a nice, rich, creamy texture?