As kids, we used to grow a ginger beer "plant" up from Ginger & Sugar and a few Sultanas. Every couple of days we would add a bit more sugar-water to it. There were no real measurements, but I know for sure we did not add any yeast other than what was on the skin of the dried fruit. (Obviously if it grows a chunky black pellicle, you don't use it.)
Many recipes for ginger-beer plants on the internet include adding a teaspoon of bread-making yeast, I really think this makes the whole production of the "plant" pointless, as you can just put the yeast directly into the ginger sugar-water (~wort).
Generally wild yeasts (i.e.: from sultana skins) are reported to ferment with Saison like characteristics. Of course there would be some bacteria in there too - brewing as 10-year old kids, there probably was not any real sanitation practices, sure we used a clean glass, and maybe washed our hands beforehand.
However the alcohol tolerance with wild yeast is unpredictable. Typically bread yeast finishes out at approximately 14% AbV (or at least it does so in meads). Whereas wine yeasts, particularly champagne will stop at higher levels. Fermenting at higher temperatures will also produce many more esters (flavours) from the same yeast strain.
I think one of the best methods to make soft-drink (soda) ginger beer is to use PET bottles. Twice a day squeeze the bottles. Once they have become hard (with CO2 pressure), put them in the refrigerator. It gives you an idea when they are good to drink, and reduces the amount of alcohol to the minimum needed to achieve carbonation. Note: this only slows the fermentation, it does not stop it. Or if you're going for high-alcohol, just release the pressure and re-cap.
