My best suggestion would be for them to experience the Dark Ages from inception . . .
First they work as security forces for in Prefecture IX though you have to be careful about the set up as the Republic frowned on mercs inside their borders. You might make the facility being guarded something like a mech final assembly or perhaps the Prefecture refurbishment depot. Make it a mix of infantry, armor and battle armor guarding the facility and when the Blackout occurs there are four or six mechs and as many pilots.
The security force goes on protecting the facility for X determined time until a group of raiders jumps in. Whatever world of IX, the security force ends up integrated with any planetary defenses available- say the planetary police who find some PALs in the back of their warehouses, noble guard infantry, even converted armored cars no longer picking up cash from stores. Win, lose or retreat whatever happens . . .
Before the second raid, the group becomes a bit more integrated with perhaps some of the other forces in the defenders joining the proto-merc unit. Still guarding the tech facility they might upgrade some IndiMechs they take in as well as using them to train new Mechwarriors. Next raid will be a different group, perhaps an appearance by some of Bannson's Raiders trying to clean the tech site out. The staff is going to be important to the proto-mercs, specialists at refurbishing machines will help them form. The forming unit is either made to feel unwelcome or abandoned by the planet- perhaps they decide to flee when the local government tries to take over their equipment. The collection of battlemechs, IndiMechs, armor, BA and infantry all board a cargo dropship to leave the planet for Galatea.
The group officially forms a merc unit enroute to Galatea, signing papers and setting up an official unit. Perhaps throw a training match at them before they move off back into Prefecture IX for a garrison contract . . . or perhaps they got hired as raiders by someone else trying to get what they need to build their own personal empire. This can keep up until you get to the timeframe where the Falcons arrive with their desant. Now desperate worlds are throwing a lot of money at the unit- question is have they taken in enough salvage and skilled warriors to survive facing the Clans?
They can continue on in Prefecture IX until the Fortress Wall goes up which should cause a lot of fun panic and civil issues. Raiding becomes more rampant and systems start to form coalitions. They can be involved in brush wars between groups. Then the Crusader Wolves start to run through Prefecture IX, and now the whole prefecture units to attempt to hold off the Wolves.
After the Wolves move through- I think- the Falcons start to snap up worlds and the Lyrans reclaim some of them. Question is does the unit retreat back to Galatea, answer the Marik-Stewart Commonwealth's call for mercs or perhaps enter Oriete's employ as the FWL starts to reform.