I do not use Peacock myself, but I'll list what assists I'd recommend for you to use to help with her zoning.
If you want to use an assist to improve your zoning game, any assist that hits across the screen is what you should look for. Filia's Hairball, Big Band's Brass Knuckles, etc.
If you do decide to change your assist with Cerebella, I'd recommend Lock N' Load. It will help you keep the opponent out of your face.
Cerecopter doesn't have much defensive use, and is used more for offense and pressure.
Robo-Fortunes's best assist for Peacock would be her heavy version of the beam, which is another projectile that should cover your opponent's approach from the air.
If you go to 3 characters, you need to keep in mind that Peacock won't be as strong. If you do use three characters, out of the characters you have listed, I'd recommend Lock N' Load for Cerebella, and Napalm Pillar for Parasoul. That way, you'll have Lock N' Load to help your zoning, and Napalm Pillar to keep them away if they try to approach you.
I'd say to stick with two characters if you want Peacock's zoning to be the main point of the team. I think she works best with only one other assist, because having two assists doesn't really cancel out the less damage she does. It's up to you, though. I'd say play with whatever you find most enjoyable.