I replied in your other thread too.. But you'll be fine if you leave Co2 in the tank for extended periods of time. Just don't leave it anywhere near a heat source.
Also, before your tank gets filled next they will empty the tank entirely anyway- so don't worry about that.
You can alternate between Co2 and Compressed air on most guns- but you need to rechrono the gun after switching air sources as Co2 is unregulated and your speed will be different. (be sure to check your manual or with the manufacture of your gun before using Co2 on it if you're unsure whether its safe to use or not). Every gun that can run on Co2 can run on Compressed Air.. The inverse is not always true though.
You CANNOT alternate between Co2 and Air in the SAME tank. NEVER try that. I had a customer come into my store a few weeks ago with a Co2 tank that a scuba shop had filled to 1500psi with compressed air (for his bb gun) and that scared the **** outta me. Very dangerous.
Thankfully that guy purchased a paintball compressed air tank for future use on his bb gun, and he let me drain that Co2 tank before someone died.