Fruit juice is a naturally occuring substance that is pumped out of a poor unsuspecting fruit. Soda is a formula that is man made much like motor oil. Some sodas are nasty together, much like some oils would not work together.well.
Yes, by all means, read the lable. "Fully compatible" but "may" not protect for extended OCIs.
I have tried to get a well considered answer to this one too. "Mix away" or "engineered with balanced additive package". The replies will be from one end of the spectrum to the other. You can find some great UOAs from mixes. It can work. Sorry for the rant. I know that I don't know the answer to this one.