yes they are refillable
Yes, in-fact only pin valve tanks are refillable. Ones with a non resealable top can't be refilled.
drain the tank, throw it away
You must have them hydro-tested or throw them away. You will not be able to fill them legally.
Paintball guns can use Co2 from any company, they are all built the same. You can find valved tanks and 12 gram's at sporting stores, paintball fields and proshops.
Call them and ask!
At a paintball or sporting goods store.
No, they must be filled (and not with air). The only exception is the silver 12 gram cartridges for pistols and stock class guns.
No paintball guns are waterproof, do not submerge them, even partially in water.
They need the small sealed CO2 cartridges, not the screw on tanks. but yes they need Co2 to function.
size is all personal preference, you can get something as small as a 9oz or up to a 24oz co2 tank, the larger the tank is the more shots you will get off of it but the heavier and larger it will be
All paintball guns require propellant, but some use HPA or high pressure air, instead. HPA tanks are more expensive and holds less shots, but are significantly more consistent (accurate) and easier and cheaper to refill. You can also tell how much you have left on a HPA tank.
Nearly everything. Paintball guns are fired through the use of air or CO2. Also they shoot only paint. bb Guns shoot pellets, and look completely different then most paintball guns.