I would assume that the water caste would know as much as they had learned by observing the other castes they worked with.
Bet there comes a time when the Tau have fought the Imperium for so long, world after world, when they start to realize the depth of their own infant stellar empire. Like Farsight, but for EVERYONE. I'm gonna base this on him knowing the name of the Imperium, but not the full scale of their size. Just that they are powerful, alien hating, territorial, war mongers.
I'm thinking about doing a small reboot type thing, moving it forward to the end of the mission, promoting those who are at that point and forming an elite group with those players who wish to stay.