Looking at modern organization, a Marine Expeditionary Unit seems to be the basic building block.
It consists of a reinforced infantry battalion, with aviation (helicopter assets), a battalion-sized logistic element, and a command element.
Troop strength is approximately 2,200 and is usually commanded by a full Colonel.
The ground element contains an infantry battalion reinforced with an artillery battery (with 6 towed 155mm howizters), an amphibious assault vehicle platoon (of 15 vehicles), a combat engineer platoon, a light armored recon company (around 14 light tanks), a tank platoon (4 MBTs), a recon platoon, and an anti-tank platoon (with Humvee mounted TOW launchers). The recon platoon includes Force Recon Special Forces. Total strength of around 1,100 troops.
The aviation element typically has 4-6 attack helicopters, 3 utility helicopters, 12 medium lift assault helicopters, 4 heavy lift assault helicopters, 6 AV-8B Harrier jets, and 2 KC-130 refueling aircraft, plus technicitions, ground teams, ordnance specilists, flight controllers, etc., etc., etc. with a total strength of around 600.
The logistics command element has all of the specialists and equipment necessary for the MEU to support itself and sustain operations for 30 days in combat. It includes supply, maintenance, military police, EOD, fuel transports, internal comm, medical and dental, etc. With about 300 personnel.
The Command Element consists of about 200 Marines and includes the commanding officer and his staff, plus naval fire support teams, recon teams, surveillance, long-range comm, SIGINT, electronic warfare, intelligence/counterintelligence, and public affairs personnel.
Now, for the Southern Cross, we could have three or four of these Marine Expedition Units comprising the 1st Marine Division (6,600-8,800 personnel, plus command and support (430-500, getting us to the 7,100 lower number limit or an upper limit of 9,230 for 13 ships) . . .
OR we might take a look at a modern
Marine Expeditionary Brigade.
An MEB is commanded by a Major General (who in the Army commands an entire division) and is built around a reinforced infantry
regiment, an entire Marine Air Group (40 Harriers and 24 Hornets, plus choppers), and supporting elements with somewhere around 14,500 officers and men. On the down-side, that would require 21 ships to transport (for a total of 101) with another 1,800 crewmen, which is a bit large.
Personally, I would lean towards having three (or perhaps even FOUR) MEU size units organized into the 1st Marine Division. Three would take 10 ships, having a fourth would add another 3 (total of 90-93 Aquila).
Of course, we have to translate everything into BattleTech terms, which I am working on.
MA