I gotta go with Nameless2All on this one.
The Tau have some of the most advanced tech known. Air Caste would have ZERO problem in normal gravity given a special suit. Lets not forget the battlesuits that Tau have. The idea that the castes cannot cooperate with each other is ridiculous. They do so on a regular basis. If they didn't, they couldn't pose such a threat to all the other races in 40k. Having an Air Caste in a light armoured suit that protects them from gravity and handing them a pistol is no different than running any other tech or knowledge savvy archetype. Water castes would be fellowship based. Fire Caste would be ballistic skill based. You would have inducted species fill other roles as well. Inducted options also allow imports from the other books. The mere idea of running around with my Greater Good group inducting other planets through negotiation or violence alone makes me want to play this game.