Thermos do not thrive at medium-low temperatures (85-90F), so you may not see much of a pH drop prior to rennet. This is especially true if you are using a dry culture instead of an active Mother Culture. The dry bacteria simply aren't going to do much at all until they reach an optimum temperature. That's why some recipes actually say to add rennet from 0-10 minutes after adding the thermo starter culture. Unless the bacteria are actively multiplying and converting lactose to lactic acid, there isn't much point in holding Thermos at medium temperature. Once you begin the cooking phase and raise the temperature to 110F or more, the Thermos will often kick in with a vengeance.
With my Gruyere, I use a little Aroma B (meso) along with Thermo starters. The Aroma B has marginal population growth from ripening to the beginning of the cooking phase. Then the higher temps will kill off much of the Mesos anyway. So why do it? Well, I feel that Mesos are a part of the natural flora in traditional raw milk cheeses anyway. Although Aroma B is not a great acid producer, it does give a gentle boost in the beginning, with enough acid to improve the curd set. When the Aroma B dies during the cooking phase, the cells will release enzymes that contribute to the flavor and aging process.