What I meant by the idea of a sweet spot is the time to flocculation (soft curd set) and firm set. At a pH of 6.6, it usually takes about 20-30 minutes to set the curd. With a pH of 6.2, it's more like 8-10 minutes. What typically happens with the right amount of starter is that the pH starts out around 6.5-6.7, you add rennet, and pH will of course decrease, which helps to firm up the curd.
With squeaky cheese meant for immediate consumption, you want a firm curd pretty quickly, meaning you should not let it sit for too long from the time of flocculation to the cutting time. If you add too little starter culture, the milk will acidify slowly, and you'll get a curd set, but it will take 15+ minutes to soft set, and much longer to firm set. And because it takes a while to firm set, you have to wait longer to cut, which results in a softer final curd mass more appropriate for blue cheese than a squeaky cheese.
In short, no, your rennet schedule at ~6.6 should be ok in combination with the rest of the steps. It is even fine for squeaky cheese with the right amount of starter culture. The key variables are time to flocculation, time from flocculation to cut, curd size, heat time, and pH levels at each stage.
clear as mud?
Paul Kindstedt in American farmstead cheese has a great discussion about flocculation. He explains it much better than I do. http://www.google.com/search?q=flocculation+cheese
Google books has the section available. Look at pages 207-208. I got the idea of the range from seeing charts somewhere in a study that had time to curd set with various pH levels and amount of rennet. From what I recall, at 6.5 pH, it took 18 minutes to set, at 6.4 it took 14 and at 6.3, it took 8. So it's not a linear progression. I figured most people would want to see a firm curd quickly for making squeaky cheese, and then also cut pretty quickly, and decent acidification from a started helps with that.