Hitler believed the Czechs were ideally suited as subjects of a German Empire, and even believed that after a few generations they would come to identify with their new Nazi masters as they supposedly had with the Habsburgs.
Hitler publicly lauded the the contributions of Slovenes and Slovaks in Operation Barbarossa, too, so exterminating them would be a tad awkward but not beyond the realm of possibility for a crazy totalitarian state.
In short, Sorbs, Bulgarians and Croats are safe since they are supposedly Germans with the wrong language. The Czechs won't be given the Russo-Polish treatment, but will be excessively regulated and forced to put up with continual demographic interference from the Nazis who will be engineering a future in which they will be a minority in their own homeland.
Slovenes and Slovaks will be in an awkward position. Depends on how much this Germany has been uber-wanked, because in any normal situation, even a Nazi victory one, committing genocide against your own puppet states is going to be a very bad public relations move. Slovenes will be in more danger from Italy to be honest.
Montenegrins and Serbs are fucking doomed. Not necessarily because of the Nazis, but because they are caught between the Croats and the Bulgarians and a whole bunch of other Nazi collaborators from the former Yugoslavia, who will be happy to tear Serbia to tiny pieces between them.
