Fringe are free agents who could be hired by either side, that is what they did at Metalorn and as a whole the faction got more out of it. Their sith rejects are some of the stronger masters from the empire who are let and chose to leave when the empire was doing well. What is happening to Omega is the same thing that happened before when they were facing invasion, what happened at Yaga to the mandalorians. Something to rile up the faction and get its people there. That the invasion awoken a sleeping and inactive Illie with some strong masters should she choose another return. Got Sarge to reappear, got Corvo, Walsh, Jorus. The big names that are coming out of the woodwork. Hell even the Mand'alor himself is there.