Do the tau allow mixed regiments because being in war is the only function of the war caste so them being fuck buddies in the barracks is by design? Do you think fire caste woman can still take on the battlefield while pregnant, at least in the early stages? Does the fire caste breed more than the others? Are they ethnically more fertile? So many questions...
Given it's a caste system revolving around the original 4 people's that formed the tau empire im assuming it's defined by ethnicity rather than by random chance after birth.