I've tried a Mezz WD700 United shaft on my Vollmer cue, no problems there. Also no problems on a Schon cue!!!
Even the Deep Impact shaft with United joint fits.
Yes. Mezz makes standard 5/16x14 shafts, and also offer their proprietary united joint, which fits on all 5/16x14 butts, but will get slightly tighter towards the end because the male end of the united joint pin is threaded only halfway. It does clear the piloting for Schon, but not the other way around.