The Ragged Edge starts with the characters being hired by a trading corporation which has a reputation for dangerous and borderline-legal jobs. Pretty quickly, they realise that they're working for what amounts to a private espionage agency.
So, the characters need to be free to join a corporation in some fashion (no Officers who are currently enlisted in a standing army or navy, no Psi Corps telepaths) and need to have a degree of flexibility (a Narn who absolutely hates all Centauri and will never ever deal with them won't work, for example). There is scope for double-dealing later on (for example, a character could be working for the Rangers, or Psi Corps, or EarthForce Intelligence as a spy inside Autumn Shipping).
Any sort of 'independant' character - a mercenary Soldier for hire, an Agent on the run, a rogue Telepath, a Trader, an exiled Narn... Autumn Shipping operates throughout known space, and employs all sorts of races and specialists.