Within the cannon for the fed sons you have 50+ regiments or so that have been mentioned in various sources. Actual cannon names can be found in fed sons source books or on Sarna net and various mentions in the books. You want to play as the FED SUNs then fine you pick one of those named regiments and away you go.. If the name you want is picked then you could pick another or Join the group that have that name and become there second or third battalion. There's no rule about how many battalions a Regiment can have so you can keep making as many as you like. Or you could Make up a new regiment, but make it so it fits with the existing named units.
The Merc's can work the same way, if you look at cannon sources Wolfs dragoons had 5 regiments and several independent Battalions, there were 2 regiments of kell hounds, 5 named Northwind highlander regiments. The options are endless.
And the devs well there the First prince or the Captain-General or his Cestral Wisdom the Chancellor[color="#000000"].[/color]
The only thing i would say to those people who want there own identity is remember your playing in a universe that people spent many years creating and major parts of the life and souls are ingrained in the Books, history and stories. Don't dishonour there hard work Or the universe they allow us to play in buy coming up with stuff that destroys the feel of the Battletech/Mechwarrior universe.