Interesting concept but would be really hard to manage lol...
If you wanted a pack that feeds paintballs, you would need a pneumatic or electric conveyor for the feed system. You could use an SC shell for this. It's cheap and it's a pack shell
You could also run a remote coil from the gun to the "gas" tank (depending on the air source that you decide to use [compressed air, nitrogen, or CO2; gun types vary per type]) which could be mounted to the shell in a TVG fashion. Paint the tank green to look amazingly similar to a tank of slime. Honestly, the sky's the limit.
Helmets/masks can be airbrushed and clear coated to look like, say, Stay Puft's head, or Slimer's head.
You could have scenario games with something like STAY PUFT vs EVERYONE and have "Stay Puft" in a fluffy suit. Only way to tag them out is to hit them in a "kill spot". Or 4 Ghostbusters versus all ghosts. Ghostbusters get a full hopper, ghosts get 10 rounds each. Think of the paint balls with ghosts as slime; so you'd want purple, pink, green, etc... The orange paint balls could be Ghostbuster stream colors. Again, the sky's the limit!
I think getting everyone together wouldn't be that bad but the best ay to do it would be to rent out a field for an entire day and it be closed to the public. If you get the amateur - professional players running about, it would just kill a fun time.