I think the grunt group "rules suggestion" is solid in the hands of any fair GM with a modicum of sense. I absolutely despise the fact that a rulebook has suggestions in place of hard rules, though.
When grunt groups because bad is when one of two things happens. 1, the individual NPC's in question literally cannot possibly be a threat solo. 2, when a GM inevitably ignores the "suggestion" of not making the grunt groups larger than 5 members.
I can seriously see it now:
"The sea of invalid, wheelchair bound lepers slowly and painstakingly roll upon out of the dark night! Surrounded on all sides, you witness the grimaces of pain upon their faces turn into dark sneers, and dark urges. They advance upon you, one terrifying mob, as grotesque phlegm, raspy voices, and the bludgeons of countless wheels overcome you! You should have never taken that old man's meds. . .".
Here's my suggestion - write hard rules in your core "rulebook", not suggestions. Let individual groups sort out their house rules and suggestions.