Ye Olde Inn

When defending, what shield symbol do the mercenaries need?

I have been playing that they need white shields to defend when on the side of the player, and monster shields when being controlled by Zargon.

I looked in the rules, but might have missed it. Am I playing it right?

That's how I would do it. Monsters use black shields, heroes use white shields. For purposes of categorization, those on the side of the heroes are like mini-heroes and those on the side of Zargon are monsters, regardless of if they use the same model.

Yep, that's how I do it.
I look at it as WS is 'goodie' and BS is 'baddie'!
The term monster can be misleading when refering to chaos figures, as there are several that are Human/Elf or Dwarf.