There is no particular compulsion to say Radyallahu anhu"Radyallahu anhu" or karam"karam Allah wajhahwajhah". Even Alaihis"Alaihis-SalamSalam" (may peace be upon him) is fine. There is NO religious compulsion on saying what with whom.
It is simply a matter of conventions which can certainly be overridden with any other word which does not contradict the essence of Islam.
May peace be upon you too.
