Most of the systems I've seen with mooks have one of three modes:
1. Fully functional monster/character, but any damage takes them out.
2. Fully functional monster/character, but with 1/3 to 1/5 normal HP; groups may act using the help rules
3. only groups functional, usually low HP as per type 2, but ability to function degrades as individuals dropped,
Mode 4 I've never seen alone, but it's a special case in a few:
4. Any crit takes out the NPC, as does sufficient cumulative.
FFG Star Wars is group 3&4 for minions, but group 4 for rivals. Nemeses function exactly like PCs.
FFG L5R is borderline group 2 and 3. It's also group 4. Minion types are unskilled, unless in groups. Rival level are taken out by any crit. Excess damage causes crits.
In both, minion groups take excess damage from one member to another member.
7th Sea is group 1.
D&D 4E is group 1; 5E has an option for group 1.
John Wick's Blood & Honor makes mooks type 3.
WFRP 1E has a sort of backdoor mook rule... The conversions to WFB 4e. If you run ordinaries using your WFB stats, and notables using WFRP stats, you get the one-hit wonders that still present a real threat. It's not the intent, but I've done it, and it works well enough. Mook vs PC uses WFB to hit, but then do the damage roll as WFRP. PC vs mook? Use conversion of PC's stat to WFB. (Given the high potential for 2 attacks, and the better hit chances, 4th career PCs are often +15 or better models...)