Have you tried the Lethal Necrotics SNPC zombies? I've seen them work really well on NS RP servers I helped organize/make, they actually have the NMRIH zombies in there too but better. Sure, the Combine zombies may be a bit of an issue for example but we can just not use them and only use the ones that do indeed make sense to be used. If you're looking for what I suggested here it is (It's a collection of all the Lethal Necrotics SNPC add-ons. I'm not saying to get all of them of course as that wouldn't make sense since there is TF2 zombies but still, I'm sure you get what I'm trying to show.):
https://steamcommunity.com/workshop/filedetails/?id=1887994096
