I have a max level druid and I am leveling a monk. Both leather. If I collect a leather piece and equipe it to the monk, will both characters have access to the mog of that piece?
I know class specific gear would be the exception, but what about generic greens, capes, weapons, that I loot via the leveling process?
Yes it will, I have collected cloth mogs on my dk and they show up for xmog on this lock
This is news to me. Recent change?
Been a thing since the transmog collection came out in 7.0.
I have yet to put on something non-plate on my warrior and see it unlocked on my other characters of the appropriate armor type.
A monk and a druid share all leather appearances (as do rogues) that are not class or faction restricted. So for example while you need an Outlaw rogue to do the class to get the Defias Leather ensemble, every leather class can mog it. However doing a mog run in Blackwing Lair on your druid and picking up say, Bloodfang Hood will not unlock it for your rogue.
AFAIK, that never happens.
A character can only add items to a transmog collection that its class is defined to use. Even if they can wear the item, if it’s not their defined armor type, it won’t become available. So, in your example, a plate-wearer will not add cloth, mail, or leather to a transmog collection even if they put it on. Plate-wearing classes are defined to require plate, so they only add plate in transmog.
Beyond that restriction is also class-based restrictions for class-specific (including old tiers) armor items.