A Mook always has crappy weapons. They have no favored arenas and no special effects.
A Mook only gets 1d10 to attack. Unless they’re attacking as a group. In that case, roll a number of d10s equal to the number of Mooks in the attack, adding together the best two.
Mooks never have face dice.
Mooks only have 1 hit point each. When you do more than 1 damage to a single mook, you kill a number of mooks equal to the damage dealt. Assuming there are that many mooks in the arena.
The primary reason I wrote these rules was so I could have one big-bad villain or monster accompanied by a bunch of lackeys, without having to roll separately for the attacks of each of the lackeys. I think the rules work really well for that use. But let's take a moment to compare the mooks to monsters and regular NPCs.
A single mook is just waiting to be killed. With only one die to attack and no special weapons, the poor guy just can't dish out any damage. He needs friends to help out.
Five mooks have the HP equivalent of a regular NPC. They still don't have cool weapons, so that means no favored arenas or special abilities. But five mooks attacking the same enemy means 5d10 to attack. That's an average roll of 18, which hits every armor class. Or... the five mooks don't necessarily have to attack the same enemy. You could have two useful attacks, one at 2d10 and the other at 3d10. So, in this case it's pretty clear that five mooks can potentially do more damage than an NPC with similar attributes. But, as the mooks take damage, they loose their capacity to return fire. The NPC does not.
When you take those thoughts and compare 10 mooks against a monster, then things get really interesting. Now, mooks don't get powers and monsters should have WAY MORE attribute points than any mook ever could, but what about the ability of 10 mooks to make 5 attacks? Or two 3d10 attacks plus a single 4d10 attack? That's pretty solid. Mooks can definitely start out dangerous in the right numbers.
So how do we take the mook rules and apply them to a monster that can turn into a swarm of something?
After careful consideration, I wouldn't. I cannot think of any way to apply those rules to a monster without making it uber-powerful. Every time I try it turns out like crap. And I've been trying to write something like this since the first iteration of the game. It was a monster power called "Plural" back then, and it was pretty lame.
So, instead, I offer a different idea for a monster who can become a swarm of something:
Swarm - This monster can turn itself into a tight knit swarm of smaller beasts. While in swarm-form the monster has +1 Armor Class and treats any arena like a favored arena. The monster may not assume swarm form when it has 5 HP or less. The monster automatically fails all Eloquence rolls when in swarm form.
It's more complex than the other monster abilities, but I think it's on par with them, power-wise. What do you think?