Assuming you mean in the major battles, most of the orc forces have magi of some description or number (although note magic-utility of orcs above).
While PD do like organising their monsters by nation…
(Ah, you’re the Dawnish monstering. Can you bring lots of clank and play heavy assault infantry, please? And the Urizeni, good, can you bring your staffs and play a battlefield ritual team?)
…because that does lead to monsters performing roles they’re familiar with, I don’t think there’s an issue to you going “cross-nation” for monstering. So if, in the above example, you held a hand up and asked to join the shieldwall troops, or the battlemages, or the healing corps, or the non-combat water carriers, your response is likely to be “sure, they’re over there, talk to THAT ref. Next!”
In skirmishes and monstering for quests, there may not be available barbarian mages for you to play.
Note also that the dedicated Monster Crew may well get passed any complex and detailed monster roles in advance. So if the orc battlemages have complex briefs and specific costumes, you may not get to join in at short notice.