You can manually summon all the zombies
/summonzombie MultiZombie
/summonzombie SpeedsterZombie
/summonzombie Boomer
/summonzombie SuperZombie
/summonzombie Tanker
/summonzombie Thrower
/summonzombie Catcher
Config File:
You can edit the spawn rates , catcher power , and max range of zombie spawning
Warning: If you increase the spawn rate more than 15 it may cause the server to lag.