I have done many beers in this fashion over the years, with brett added in secondary as well as used strictly for primary fermentation. It really depends if that is truly the route you want to go. If so, once primary fermentation with your abbey yeast has slowed down (10 or so days), you can rack it to a smaller secondary carboy (typically 5 gallons) and pitch your brett (no starter needed). Strains that work well in this secondary process include Wyeast Brett Lambicus (will give a slight cherry pie aroma), Brett Bruxellensis (typical horsey, farmyard), and Brett Claussenni (tropical fruit, hay, and earthy characteristics).
Be prepared to give this beer anywhere from 4-6 mos to allow the brett characteristics to come through in flavor and aroma. Let your palate be your guide while trying to minimize samples from the carboy. Take your first one at 3-4 mos so you can get an inkling as to how the flavors are changing.