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But aren't the Tau the real communists?^^

Lol, good one.

Also I don't think the Tau caste system and communism exclude each other.

Because, yes, each caste does have a specific role in society and changing of castes or cross-breeding are not possible, but that is mostly due to actual genetic and anatomic differences between them and, what is most important, each caste is equally accepted and belonging to a specific caste does not determine the social status (except for the Ethereals, but they are radically different from the other castes anyway).

Every caste in seen as equal because they all contribute equally to the greater good.
So the Tau caste system in actually pretty different from the traditional image of caste system in real history, where the caste also determines the social status.

That means calling the Tau communists is actually pretty accurate. In their society everyone is generally equal, the castes only divide them into the specific roles they perform for the greater good, which is also quite effective.

And the main point of their society is that the individual does not work for himself, but for the benefit of their whole race (or the "Greater Good"), which sounds very much like communism for me.

-The Tau live in a cast-based society so they are obviously not communists.
-The eldar are too different from mankind so i don't think any form of goverment invented by humans can be used to describe the eldar society.
-The forces of chaos are slaves to the dark gods how are they supposed to be anarchists?

orcs are more like war lord type systems. not complete idiots as there is some form of order. but otherwise, yes.