Is it permissible to write or say alayhis salaam after the name of a Sahaabi who was among the Ahl-e-Bait (the Family of Rasulullah – sallallahu alayh

Answer

Since it is a salient feature of the Shiahs to use the invocation of alayhis salaam for their Imaams, etc. it is not permissible to emulate them. If the Sahaabah are from the Ahl-e-Bait, the appropriate invocation will be used for them, namely, Radhiyallahu anhu'. If they are not Sahaabah, then Rahmatullah alayhi will be used.