This sounds interesting, do you plan to have them as a faction that already has a foothold in "our galaxy" or do you plan to have them as a force that is just entering it, planning on conquer but with only a limited number of troops, like some advanced scout group or something?.
I plan on having an expeditionary force that comes to the Sol System some time during the Colonial Civil War. Much like in the fashion of the Achemenid Persian Empire, they send an emmissary after studying their culture and language for some time, and offer them a place in the Empire without violence. If they refuse, then they will be conquered anyway. But...
I also kind of like the idea of having corporations which are more like "mega businesses" (although i'm not sure if that's what you meant).. Are these 5 groups the sole factions or would you plan to include others? Or maybe some native factions, i.e some creatures or species that are native to titan and oppose the Titan Federation. I'm not asking these questions critically or anything i'm just curious. All in all is sounds very interesting
Megacorporations or syndicates is the proper term. But yes, you are correct. Business run everything on Earth and other colonial entities.
And yes, these are the only 5 major factions, but they all have several military sub-factions beneath them. Like the Titan Federation has many federal subjects that all have their own version of uniforms, weapons, etc. Kinda like how the 40K Space Marines are one faction with their chapters with different doctrines as sub-factions. The Sons of Liberty, Skoll Empire, and Corporate Alliance all have sub-factions as well.
The Sons of Liberty all have different cells. The Skoll Empire operates different cults. The Corporate Alliance has their myriad of Mercenary Armies. The Martian Commonwealth does not have different sub-factions per se, but they have an extremely large military force that is modeled after the Waffen SS and the People's Liberation Army. I guess you could count the main army as one sub faction while the military arm of their political party as another.