I've been putting together an AU for a late 3rd Succession War/early 4th Succession War, and have been tinkering around with organizing the military of each nation.
For the FedSuns, I'm currently working on the Crucis Lancers. I'm toying with the idea to make them the main planetary assault force, so wanted to organize them accordingly.
What I'm thinking -
Reinforced Battlemech Regiment - 4 Battalions (Heavy and Assault designs) - 144 'Mechs (not including command elements)
Aerospace Regiment - 3 Wings - 60 Aerospace fighters
Vehicle Regiment - Vehicle Recon, Artillery, Air Defense. Working on numbers here, but want to be able to give each Mech Battalion support
Infantry ??? - I'm thinking a brigade sized jump infantry unit. There would be VTOL transport, and supported by VTOL gunships.