Our course did the abinitio on the 172rg w constant speed and retractable gear. I never noticed any problems for me or any of my classmates relating specifically to the relative "complexity". I always wondered if maybe we got an easier path then the ones having to relearn after half the course.
To me it seems highly likely that it is not a major issue to start with constant speed from the beginning (considering that the course/instructors has to take it into account -ex. good standardisation between instructors). Considering the total amount of impressions and experiences the student has to process to learn to fly from scratch 1 or 2 controls more or less is not such a big difference. Those extra actions only stand out for the student that alredy have habits that needs retraining (switch between fixed - constant)