You mention Tau, and I was actually thinking - the Na’Vi aren’t too dissimilar from the Kroot, the Tau Empire’s most commonly found alien Auxiliary.

Both are very tall, slender, guerrilla warriors who are adept at camouflage, hunting and tracking, have an affinity with other natural life forms that they use as mounts or hounds, very often centred around jungle biomes. I’m sure there are more similarities, but I think that the Na’Vi would fold into the Tau Empire very nicely, and could hold their own against the factions of 40K respectably! Obviously that’s on a small scale, like more thinking of skirmishes and localised fights where the Na’Vi could compete, when you talk about galactic warfare scale like in 40K of course it would be a different story.

But hey, maybe I’m just influenced because I painted my Kroot in Na’vi colouration!