That is an inline on/off (looks like it's for a remote of some sort). You still need to attach it to your tank, and find a way to mate it to a CGA320 fitting that comes on the regulator. (and by no means is a paintball cylinder a modified CGA fitting)
I assume you are in the business. I would have personally recommended just getting a regulator from palmers pursuit shop. They have regs that attach to a paintball cylinder and output 1/4" hose to recharge draft beer. They are about 100$, but that is the reg aswell.
I normally would have been paying 5$ for a fill regardless of what size tank I have, but I use HPA and it's also free fills at the local field.
a 10lb tank costs about 10-15$ to fill and lasts 11+ months. (That is the equivilent of 18 fills of a 9oz bottle. and I don't have to run out every week or 2 to get one filled.)
To each their own. If you are buying the stuff to inject Co2, why not buy the right type of stuff..