There's two critical types of information being posted here - the quality/hit/feel/preference of a Mezz shaft on a Schon, and the mechanical fit of it.
The Mezz United joint is 5/16x14, but with a straight piloting. I've found this to fit ever 5/16x14 joint I've ever come across, including Schon. I've never noticed a difference between United and non-united Mezz shafts of the same thread pitch.
Schon uses a different 'half donut' shaped piloting, which fits exclusively to Schon butts. You can get non-Schon shafts made to have a Schon pilot (e.g. Predator 314), I generally do not consider this to be wise if you aren't certain you'll keep it or sell it with the butt.
At the joint, you may notice an ever so slight mismatch on the order of a tenth or hundredth of a millimeter. I've seen combinations with, some without (or beneath perceivable limit).
As far as hit, I am a fan of that order of combination of brands. I find Schons to be a little more back-weighted, though Mezz lets you add weight bolts accordingly (Schons do not from a user-adjustable standpoint). My experience with the WX900 was much more delightful than the Z2/Z3/OB Pro Plus from a 'crispness' score.