Please remember that these are rumors and to apply the normal amount of salt.
via Jared Van Kell
An earlier rumor
Tau Ethereal - Unlocks Tau Firewarrior teams.
Tau Fire Commander - Unlocks Tau XV8 Crisis Battlesuit teams.
Kroot Shaper - Unlocks Kroot Carnivore squads.
Demiurg Ancient - Unlocks Demiurg Warrior squads.
Jared added this clarification
From what I understand they become troops. However I have a strong feeling that firewarriors are going to be remaining as troops that can be taken irrespective of what commander you taken but that the different commanders allow you to take other units as troops instead.
With regard to ethereals I believe that they will allow honour guard to be be taken as troops (with BS4 and preferred enemy) or at least allow firewarriors to be upgraded to honour guard.
For the kroot carnivor squads and demiurg warrior squads and indeed the other kroot/demiurg units as well I do believe the rumour is correct in that you will have to take a shaper/ancient to be able to take them. For Crisis battlesuit teams I am sure taking a tau commander they are moved from elites to troops instead or at least become scoring.