When do you rack to the secondary? Do you let the primary yeast finish completely first or leave a little extra for the Brett to chew on? One idea I had was to rack to secondary when it gets to the 1.020's to possibly slow down the saison yeast a bit while the Brett gets going.
Allow your primary yeast to complete a healthy fermentation, take up by-products, and floc out.
Brett doesn't need fermentable sugars (or even unfermentable ones) to live and produce flavor compounds. Brett will metabolize just about anything and turn it into flavor: proteins/carbohydrates, hop acids, even alcohol and existing flavor compounds from the primary yeast. Brett will have a flavor impact despite the FG, especially with that specific strain.
You'll need a few gravity points for brett to ferment and produce CO2 (I assume you're bottling?). I always assume that brett in the bottle will take me down to 1.002 eventually, take that into account when you're thinking about priming sugar. You can also bottle some without priming sugar for long aging. Prime the rest like normal, opening a bottle every month or so to check for over carbonation.
You don't need to make a starter unless you want to keep some brett slurry around for later on. I use brett in a lot of beers, so I keep a starter going and pull off slurry as I need it.