Certainly Orcs don't appear to be affiliated with either side and are an enemy to all, it's possible a huge Orc army will force the two opposing factions to unite to face a common enemy.
Both black and cave goblins seem not to stand for either faction.
We have only seen some groups of Orcs occuping fortresses, but Tinkerer hasn't revealed how structured their Race. So far we know they will annihilate other tribes males and take their women, or simply kidnap any female they find.
Each Orc group seems to be an Outpost of a bigger force.
The Goblin Tribes and the Gumpling are similar. Each tribe within a race is independent and only care about their own tribe. If they can, they will make other tribes submit and take the women. Or annihilate only the males.
I think the Orcs and each Goblin tribe only stand for their own people, and are not part of either Faction. Their are independent groups that could join the Nergal faction if they see a benefit.
Humans, Elves and Giants have had problems with other races (Orcs, Goblins, Gumplings), but they were conflicts with an specific group each time.
I wouldn't consider this humanoid groups a common enemy of both Factions. They have been bothering the Qetesh Faction; and the Nergal Faction ignore them or see them as potential tools/resources/minions/allies.
