Proper Communism is Egalitarian society with Utopian living standards.
On the other hand Totalitarian Dictatorship with abolished private ownership could be cool, but Stellaris cannot emulate something like this.
BTW, why everyone so focused on Militarist ethos for their conquest as Authoritarians? From all PDX games (well, maybe aside from HoI) i got that it's all about securing your conquest, and not about conquest itself. Basically stopping the unrest on new territories. How Militarism help with that? The only thing that comes to mind is more effective\cheaper land armies to control unrest (and related tech to show more often), but that's all. The core is stronger economy.
If you're going for conquest using Vassals, i really see no strong reason to go Authoritarian. You can do it, but other ethos fit there better than Authoritarian (actually, in this case F. Militarists does make sense)