Wow, crazzydave is getting graped and AlCophile is getting bad info from AirGas. :cross:
Beer Gas is a CO2/Nitrogen mix most commonly used in stouts where you want the cascading effect and a very thick head. It's 100% NOT necessary for use in dispensing 'heavier beers'. Beer gas is used AFTER a stout has already been carbonated with CO2 since the nitrogen level (typically 75-80%) will prevent CO2 from entering the beer in keg.
Have a balanced system (which works for the pressures you'll be using) will result in NOT getting tons of foam for all beers. I use CO2 for my porters, browns, pale ales, IPAs, etc. Basically ALL my beers are put on tap and carbonated at a CO2 pressure set that works best for the style. Figuring that out is key.
A good many of us have found that fire extinguisher shops (those that recharge the CO2 ones) are a GREAT source for CO2. The 'purity level' of CO2 isn't an issue from these sources, since they're within a few thousandths of a percentage of the one the AirGas managed told AlCophile about.
BTW, I paid $17 to get my 20# CO2 tank filled at a fire extinguisher place. They fill up 5# tanks for $10 and 10# tanks for $12.
IMO, the AirGas guy was trying to scare AlCophile into over-paying for the CO2 they sell.