So have it go like this(?);
BoS squad attack Four Leaf, Riley and Gunners are wounded, most of the BoS are killed off but they capture the mercs.
Jack and Brutus scout Lexington, see the Initiates and have to retreat in an opposite direction to where the others are waiting.
After a while Courtney, Rufus and Jackal follow them to see what's going on. Fighting between the Initiates and Ferals force them into the RR for shelter, where they set off a trap, meet Sparks and are discovered by the squadron.
(At some point I would like Jackal to run off and for Rufus to lose track of him, just so the Initiates don't shoot him on sight.)
In Bunker Hill the rangers spectate the arrival of the Minutemen, and could learn about the political situation in the Commonwealth. They may well side with the MM, seeing it as a relatable cause.
(Danse and Haylen?) Could send out a transmission ordering all BoS forces to Bunker Hill, which the Initiates pick up on from a local radio (or perhaps Courtney's pip-boy). From there those two groups begin to head there as well for the meeting.
What sparks the standoff between the two factions, if they're there for a treaty? Perhaps one a group makes an unacceptable demand? Or the rangers might have previously mentioned the NCR-BoS conflict with the MM, who worry about a similar conflict kicking off in the Commonwealth?