I don't have a pH meter, but from what I gathered, the initial readings are usually amounts of change as Dave indicated (i.e. add culture, wait for 0.1 drop in pH) rather than "add culture wait for specific value of pH".
Peter, here's how I think of things. There are different species of cultures (i.e. ... cremoris), but just like there are different species of mamals (i.e. humans), there are different strains within those (i.e. different people). So, FD and MM100 might have the same "mamals" in them, they might have people with different characteristics - so they do the job differently. Think of strains as "clones", so using different strains is still using a human to do the job, but you're using clones of different people so they won't do the job exactly the same way. This metaphor only gets you so far I suspect, but it helps me.
Also, mixing the two cultures, even if they used the same strains and similar amounts, by adding the MM100 this would, effectively, half the amount of the Leuconostoc mesenteroides subspecies cremoris relative to the other 3.