I have been asking the staff at local coffee shops that serve nitro coffee and they have said that they usually use a blend of 80% N2, 20% CO2 and serve at 20-30 psi.
I have successfully harvested yeast from a bottle of The Bruery's Trade Winds Tripel. They use an aggressive Belgian strain that is great for fermenting beers super dry or high ABV. I know that a lot of The Bruery sours a good for souring as well since they make extremely acidic sours.
