A 20 oz paintball tank will not last you 3-4 months on a 55 gallon tank. Maybe if you did 1 bps, but that won't cut it with a 55 gallon. The fact is that you need a bigger co2 setup for the tank size. A paintball setup would work on a nano, but I would never put it on a 55 gallon. I would advise to not do the paintball rig up a wanna be regulator method. Lots of people have had them explode and it's hard to keep the bubble count consistent. Been there and regretted it.
Besides the atomic regulator all you would NEED is a diffuser. A bubble counter and a drop checker would be very beneficial but not essential. It would be fairly difficult to get the correct amont of co2 dialed in without a bubble counter.
Honestly with Java ferns and Moss I wouldn't bother with CO2. The CO2 game is fun, it explodes your growth like crazy. It also explodes your need to be watching the tank very close. Algae grows much faster also. It's a lot more work. If you wanted to grow some stems and a carpet plant then i'd go for it. If you are going to stay with moss and ferns then I would suggest to stay with no co2. moss is easier to grow with no co2 than with for me. Its very easy to fill the moss with algae under high light and co2.