I gave it a few more thoughts and here is what I came up with.
As always the idea is to be able to transport the GTIA in a single JumpShip available to the Star League and not a rare one.
The big unit will be 2 'Mech GTIA, 3 Infantry GTIA and 3 Cavalry GTIA strong with one HQ and Logistics Company (I don't know if this is enough) and a Ground Aero Wing (9 Aerospace Fighters Squadrons, 9 Conventional Aircrafts Squadrons and 9 Transport Aircrafts Squadrons).
The Ground Aero Wing should be able to move on its own but the different GTIA should take into account for their transport needs that they will be assigned a Aerospace Fighters Squadron.
* 'Mech GTIA:
- Command and Logistics (1 company (is this too much?) and 2 Vehicle Lances),
- 2 BattleMech Companies,
- 1 Tank Company,
- 1 Mechanised Infantry Company,
- 1 Artillery Company.
* Cavalry GTIA:
- Command and Logistics,
- 1 BattleMech Company,
- 2 Tank Companies,
- 1 Mechanised Infantry Company,
- 1 Artillery Company.
* Infantry GTIA:
- Command and Logistics,
- 1 BattleMech Company,
- 1 Tank Company,
- 2 Mechanised Infantry Companies,
- 1 Artillery Company.