I have used MonoPoison on PO a lot, so I might be able to help you.
Venomoth - Use 252 HP over 252 SAtt. Reason? There's no point in Venomoth trying to kill things by itself. It's Nidoking's job.
Nidoking - Life Orb instead of Focus Sash. It isn't needed if you're smart with switching and Nidoking hits VERY hard with that Life Orb boost, its recoil nullified by Sheer Force. Also, Modest --->Timid, since you've that Life Orb boost. If you don't want to change him though, he should still be a powerhouse so it's your wish.
Gengar and Tentacruel is fine.
Replace Crobat with Drapion and keep him in the 6th slot on the sim. Crobat is an amazing lead with Taunt and Brave Bird, but with Venomoth as a mandatory lead it becomes pointless. Drapion is the only Poison type immune to Psychic so use it to ensure that Psychic types, especially Jirachi and CM Reuniclus don't sweep your team with Psychic/Psyshock. It has access to Crunch and EQ, which means you can most Psychic and Jirachi down. Use Toxic or Cross Poison as a Poison move. So, Jolly, 252 Att, 252 Spe, 4 HP with Cross Poison, Crunch, EQ and Taunt or Swords Dance in the last slot. I'd recommend Taunt, since it shuts down those Will-O-Wisps and Thunder-Waves from the likes of Chansey, Blissey, Dusclops and Sableye.