What mechs should I look into for this sort of theme?
Urbanmechs, Locusts, maybe a Wasp. If you're thinking slightly heavier tonnage, a clapped out old Catapult is likely.
Also are there any established lore factions that this concept fits into?
Yes. Very yes. Lots of stories in the series are about ragtag little bands of mercenaries having adventures among the constant wars in the Inner Sphere or out in the periphery. For example, check out the origin story of the Grey Death Legion.