So I did something similar to the OP this weekend and we will see how it all goes. For big brew day I brewed 10 gallons of Pointon's Proper (you can google the recipe). In 5 gallons I pitched a lacto starter that I had at ~110F for a few days (smelled nicely sour). This 5 gallons is in a heated chamber I made where its staying 109-113F. I plan on keeping it at this temp for a few days. I am not sure how long I will keep it in there as I did use hops in my boil ~20IBUs worth. Once I decide its done souring I will pasteurize it by bring it up to 140F for ~10 min. Once cooled I am going to pitch some Brett Trois and possibly what ever bugs I harvested out of a few bottles of Monks Cafe Flanders Red. I can't wait to see how this turns out.