I believe those arrows are spined a .340....which is actually stiff for your setup, espeically with 100gr heads. They probably should have set you up with the next spine down, but stiffer is usually never a bad thing unless its way too stiff. Too weak usually will give you fits.
No idea why you made the switch to 100's....but imo, you should be totally fine with 125's. My guess is the shop set you up with lighter heads for more speed. Personally I shoot 125's in my bow (with .340 spined shafts - 28.5") at 66lbs with no issues. You might actually shoot a tad tighter groups with a 125. Do you have some you can try out and see how they shoot (groups) compared to your 100's?