Not really much advice...but I love Miniature Schnauzers...we used to have a little boy one called Sam and he was adorable.
Our Harry
(like your Zeus) was an absolute breeze to train - Sam was a different story!!!
It might have been just him, but Sam was a little bit thick...
It took him aaages to get the grasp of potty-training...he used to go to sleep at the top of our stairs then roll over in his sleep and tumble down them...he used to lay by the fire until we could smell singeing...but he was just so gorgeous and lovable
Personally I would keep them separate until the newbie is trained - Zeus has earned his freedom. Maybe a baby-gate so that they can see one another? It's not just the pottying - it'd be the chewing that I'd worry about more. It truly isn't cruel - it's protection for your new little girl-to-be!!
Oh, and I'm sure Zeus would LOVE a new little girl in his pack...
Good luck with your decision - can't wait to see photos!!