Check with your fountain supply shop or gas supplier. They usually have a preference for CO2 tanks, or Inert Gas tanks.
You'll need to get a fill with BEER MIX (25%CO2, 75%N2).
I only use BEER MIX with my Stouts & Cream Flow Ales. For this you'll need a special stout faucet which has a restrictor plate with tiny holes in it. I set my regulator pressure at 30 psi...and dispense through the stout faucet. The idea behind using Nitrogen is...you need the extra pressure to dispense through a restrictor and get the release of gases that you're looking for. If you do it correctly, you'll see that cascading head effect in the glass. The other advantage to BEER MIX...it makes it possible to pour Half N Half's or Black N Tans, etc...
If you used 30psi of CO2, you'd have way overcarbonated beer. Since only 25% of the BEER MIX is CO2...you wind up with half the Carbonation, even though you've more than doubled your pressure. Stouts and Cream Flow Ales are not supposed to be too carbonated.
The Nitrogen bubble is smaller than the CO2 bubble as well. I don't know if it has anything to do with it...but the head is creamier. It may have more to do with protiens, lipids, etc...in the beer than the Nitrogen effect, I don't really know. But it is noticibly creamier.
LASTLY...check that regulator. Does it have an overpressure relief valve? Does your keg? Bottom line..don't hook up a regulator with a set pressure over 100 psi to a corny keg that doesn't have an operational relief valve. If the regulator malfunctions...you could blow your keg up. There has to be a safety link somewhere. I know a lot of Nitrogen regulators are capable of pressure way over 100 psi...like for inflating tires on airliners, etc.. which could take up to 220 psi, etc...