No, they cannot.
Yes, they require High pressure air (nitrogen) tanks. They cannot run off of CO2.
No, but Dicks sporting goods does.
A fill station is a bulk CO2 tank, a fill hose and a scale. They are used by trained air smiths and paintball field employees to refill empty Co2 tanks. First the tanks are purged of all remaining Co2, then carefully refilled and weighed. If you are not trained on how to do this, do not attempt, at the risk of breaking the tanks burst disk or damage to you by the extremely cold gas.
The threads are different on Nitrogen tanks. There are adapters to use a Co2 gauge on a nitrogen bottle .
High pressure air, and all air for that matter, is about 78 percent nitrogen, so it is nicknamed a "nitrogen tank" . So a HPA tank is supposed to be filled with "nitrogen" which is really just air.
Firstly NOS is Nitrous-Oxide, which is never used for paintball. "nitrogen" is another term for High Pressure Air tanks, because air is primarily Nitrogen. you must buy a HPA tank to use HPA. Not only can you not physically fill the wrong tank with the wrong gas, but It would destroy your burst disk if you tried.
You must have them hydro-tested or throw them away. You will not be able to fill them legally.
You Rent big bottles and fill your tanks from that. Fine out who deliverys to your local pub as they will use the same co2.
There are many places where you could go to get them filled. Any local paintball store or field could. Like All American Field would fill them. Also, some hunting stores also have the ability to fill them.
No you don't. Tanks must be emptied at fill stations anyway. CO2 tanks can store pressure for a very long time, so you can use it for several games over several days.
There are a lot of good sites for co2 tanks. One that I prefer is DicksSportingGoods.com they have a lot to choose from in many different sizes. They also have many other things besides co2 tanks.
All Co2/HPA tanks will fit any marker. Co2 cartridges will not.