The L4D2 characters have a few extra sliders from the HL2 qc, and a few sliders have different ranges. You can even omit a few of the standard flexes since I can’t see them in use anywhere on those characters (sneer, the lesser smile, teeth-showing smile, one of the two puckers…). I model 38-ish flexes including four L4D-specific flexes (jaw_right, jaw_left, mouth_sideways_right, mouth_sideways_left, these four just can’t be modeled as two that can be split down the middle either).
Don’t forget to look up eyeballs from the Valve wiki. The tool qc_eyes for setting those up will also set-up the player/npc eyelid flexes (as long as they’re in the right order) to move with the eyes. That’s so when the character looks all the way up, the iris doesn’t roll under the upper eyelid… things like that.
Adding/changing things in the HL2 facerules/standardflex for an L4D2 character is kinda tricky, I can upload my version if you want if you but I’d like to be credited if you use them. They were a huge pain to figure out. I swear, the qc part is far more complex than actually modeling face flexes.
BTW Are you using the proportional edit mode / “soft selection” to model expressions?