aww thats easy to add monsters! ya dont need any plugin! it wont work in esf...play reg hl for this(online or single)
but ya have to be carefull usin it cuz it could crash your half-life! if ya wanna add grunts(guys with machine guns) simply type this in console: developer 1 ; mp_allowmonsters 1 ; restart
then when ya restart type impulse 76 and it should say somthin like : for grunt-o-matic restart then restart and every time you type impulse 76 a guy will apear infront of ya but be carefull cuz ya dont wanna get the guy in the wall cuz a yellow dots circle around him and he wont move(this doesnt crash it the part im about to tell ya might)
for aliens or diff guys:
cheats must be on...
Spawned characters appear in your exact position when you use the give command. You must use noclip to disable clipping or you will be stuck inside the character you spawn. also use notarget so the alien donsnt hurt ya when you spawn it(it wont follow you)
After spawning, characters will fall down to the ground or solid object below. If you are in a solid object (such as a wall, the ground, or another player) when you spawn a character, he (or it) will be surrounded by a sphere of swirling yellow particles and may be immobile. To avoid this, spawn characters from slightly above the ground (but not in the ceiling) and away from other characters.
It's easier to spawn a lot of characters quickly if you bind keys to the give command(s) for the entities you have available. For example: bind x "give monster_scientist"
Half-Life Entity Names (preceed with monster_):
alien_slave, alien_grunt, apache, barnacle, barney, barney_dead, bigmomma (gonarch), bullchicken, cockroach, furniture, flyer_flock, gargantua, generic, gman, grunt_repel, headcrab, hevsuit_dead, hgrunt_dead, human_grunt, human_assassin, houndeye, scientist, scientist_dead, sentry, miniturret, turret, ichthyasaur, leech, tentacle, zombie, hgrunt, nihalinth, osprey
hope that helps!!