Considering the fact that Nazis actually adopted Ober Ost plans for Eastern Europe in their Lebensraum projects and Ludendorff was one of their early political icons and leaders that argument seems somewhat unconvincing.
Gas chambers and death camps-that seems not likely in CP victory scenario, but ethnic cleansing, creating enemies out of ethnic groups to divert attention from trouble at home, ruthless economical exploitation, forced labour for other nations is possible(the latter two were already in part implemented during the war).
Controlling Central and Eastern Europe, along with trying to keep A-H together, or establish stability in it in break up aftermath would divert considerable German resources and prolong German military presence in the East, influencing soldiers with Bolshevik and anarchist ideology so you could see increased threat of communist revolution and a fascist coup to stop it by Junkers, aristocrats and military of Prussia in German Empire.
If Poles are given independence then others will want it too. If Poles are given Galicia then Ukrainians will hate the CP, if Ukrainians get the Galicia Poles hate the CP, if it is divided Ukrainians and Poles hate CP and each other forcing Germany to control them.
So removing Galicia from AH(which is bound to happen) will create all sorts of problem and destabilize the region.
About Mitteleuropa countries not being able to unite-the same could be said about inability of Franco-German friendship

If German Empire is oppressive enough and arrogant enough in its nationalism(now that can't happen of course

) movements like neoslavism might regain popularity and nationalists from all sides could overcome their hostility in face of a greater threat.
Plus really they don't need to be united to be a threat-discriminated and discontent population will be a great pool of revolutionaries, spies, strikers for all factions interested in defeating CP.
About Ukraine-the Ukrainian national movement was quite strong in its beliefs and wouldn't tolerate oppressive measures or discrimination by Germany. If proposed plans for control of Ukraine will go on they likely will turn against them. Also don't underestimate the pro-Russian movement in Ukraine.
About Poland-sure the pilsudczycy faction might not see any choice, but the endecja can be easily wooed by promises of regaining of Polish-inhabited areas of Silesia, Poznan, Pomorze and East Prussia made by Russian Empire. They might be not as large as areas lost in the East, but more unified ethnically and far more prosperous. I don't believe the German military and officials (which were to de facto rule Poland) often compromised out of German nationalists would treat the Polish population well enough to calm down resentment towards economic exploitation and loss of Polish territories in the East and the West. As to Habsburg puppet-monarchy in Poland was hugely unpopular in general and monarchists were a exotic minority made out of tiny fraction of land owners, clergy and obsolete aristocracy.
In general the German Empire will need to committ a considerable military forces to occupy the Mitteleuropa, stabilize AH region. Even if it would manage to stabilize the military situation in long term it would need huge economic resources to end the economic crisis in the region that happened due to war and military operations on this territory(if it would be willing to, which is far less likely than economic exploitation for solely the benefit of Germany).
Thus its power and position will be weakened by the comittments to the Empire Germany will want to create. In that situation all former Entente powers can cooperate with each other to reverse this situation. particularly Russian Empire has most interest in this, and no matter if it will be Communist or White Russia it will be formidable enough to do it with British and French backing.