Hi Maria
I did this tour last month (Sept 14-20) and thoroughly enjoyed it, all of the day trips are well worth doing.
Regarding the weather, it was fine and sunny for the first five days but unfortunately was much cooler and wetter on the last two. On one of those days we took the cable car to the top of the mountain where the rain turned to snow anyway and it actually made the trip better! The day in St Moritz was particularly hot (high 20s I seem to remember).
Hope this helps!
Michael.