Usually I just pour the white labs vial of sour mix right into the corny keg. However, I have noticed that when using a starter with either brett, lacto, and /or the white labs sour mix the pellicle forms a lot faster, and it seems to sour up quicker. Although, once that pellicle forms I try my best not to disturb it. Another thing to keep in mind, if using a corny keg I try to keep the psi on that keg at a minimum so that the brett does not have to work under pressure.