Unless you want to wait about 18 months to drink your beer, ferment your beer with your standard house yeast first. Use a mash profile that will leave some residual sugar. A 100% brett beer will take FOREVER to ferment out to dryness ( though it makes a wonderful beer ). The fruit is typically added during the secondary brett fermentation, and depending on the amount of fruitiness you are going for anywhere from .5-2 lbs. per gallon should do. Sounds like a lot I know, but it takes a lot of fresh fruit to make the flavors come through. I suggest frozen fruit as it is a lot cheaper, and quite similar to fresh in a fermentation.
Darin