Other than temperament though there is no real reason why they can't be kept together.
They share the same water chemistry.
There is no chance they will mate.
The only issue is if they will get eaten.
But with peacocks around the 10cm Mark it will be no issue.
I have now crossed referanced this information and again it comes back to temperament.
Letting the shellies get established first in the tank and then adding the peacocks might be a safer way to do it, so the peacocks aren't tempted to chase the new intruders, and so the little guys wont be as likely to flee and act like prey.