Have you tried doing something like
RegisterZombieClass("Fast Zombie",
{
Name = "Fast Zombie",
Wave = 0, -- The decimal is a percent of the total waves. Example, this would be wave 3 in a game with 10 waves total. Always rounded down.
Health = 175,
SWEP = "weapon_zs_fastzombie",
zombiemodels = { "models/here", "etc", "etc" }
Model = Model( table.Random( zombiemodels ) )
Speed = 310,
Description = "A very fast, heavily decomposed undead cadaver.@Unique abilities: Can climb walls and lunge at its victims.",
PainSounds = {
Sound("npc/fast_zombie/leap1.wav"),
Sound("npc/fast_zombie/wake1.wav")
},
DeathSounds = {
Sound("npc/fast_zombie/fz_alert_close1.wav")
},
PlayerFootstep = true
})