Except the reality is that its still cheaper to get the 5lb cylinder.
you have to find and assemble a regulator that'll hookup to a paintball tank. 5lb cylinders and 20oz tanks are two completely different connections.
Then, smaller tanks have to be filled by the right kinds of places...i.e. paintball stores. I used to play, and the price I paid was about half of a 5lb CO2 fill. So within a few kegs, you'll have to refill your cylinder, vs about 10 kegs off a 5lb.
If you have a big budget, paintball setups are great for taking kegs to parties (more efficient than the CO2 chargers that use 12-16gram bulbs), but if you want to run 2-3 kegs at a time, you want a 5lb tank (at least).