As far as I can tell, the dominant method is to pick a place/planet/city/character and then add one of the following:
Guards
Lancers
Dragoons
Fusiliers
Regulars
Irregulars
Militia
Grenadiers
Sphinxes, whatever.
Or she could adopt one of the many existing units listed on Sarna.
Emma's Irregulars? Emma's Crazy Cavalry?
Great thing about Battletech, or so it seems to me, is you can just make up whatever you want.