I wonder if the Marians continue the Roman practice of hitting mercenaries to directly fill gaps in the Legions. I'm not talking about using a merc unit to garrison Planet Y so the Legions can be elsewhere, but more like Legion A has few/no scouting units, so mercenaries are hired to fight directly alongside regular troops in that role.
It would make sense to do this with newer or less reputable mercs, because then you have regular troops right there to keep an eye on them. And game-wise, it would allow a Marian player to mix regular IS-style lances in with Marian centuries, or use units not normally available to a Hegemony player.