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At Taros the Tau had roughly 100 cadres, approx 8-9000 fire warriors, 5,000 kroot, and 8,000 human aux, they were very well supplied, and supplies were the main failing point of the IoM troops, water and ammo mainly.
Apparently these forces were placed there after the initial attack by the Avenging Sons marine chapter ( sent to kill the planetary governor )

Tau never seem to bring a lot of people. Many are under 50 and at least one is only 12.

Just a few thoughts:
First of all obviously everything said is just going to be either wild speculation or theory.

Now, just because there are four castes doesn't mean each caste makes up 25% of the population. The fire warrior caste is going to have more casualties and less opportunity to breed than the earth caste. I would expect the fire caste to make up no more than 10% of their population. Similarly I would expect the air cast (diplomats) to be smaller than the Water caste (transport), who are smaller than the earth caste. The earth caste I would expect to take up at least 40% of the population.

As for world population, I would agree with the 10-20 billion pop cap on the sept worlds. I would contend that the sum population of every non-sept world would likely not equal that of a single sept world.