Offering supplication by saying Blessings and peace of Allah be upon him (sall Allahu alayhi wa sallam) is not only for our Prophet Muhammad (blessings and peace of Allah be upon him); rather it may be said for all the Prophets (blessings and peace be upon them).
With regard to offering supplication by saying may Allah be pleased with him (radiya Allahu anhu), the scholars have determined that this duaa' is typically for the Sahaabah (may Allah be pleased with them), but if a person occasionally prays for some other Muslim by saying this, there is nothing wrong with that.
Saying may Allah have mercy on him (rahimahu Allah), may Allah keep him safe (sallamahu Allah) is a supplication that it is prescribed to offer for any Muslim, living or deceased.
And Allah is the source of strength; blessings and peace of Allah be upon Prophet Muhammad and his family and companions. End quote.
Standing Committee for Academic Research and Issuing Fatwas
Shaykh Abd al-Azeez ibn Abdullah Aal ash-Shaykh, Shaykh Abdullah ibn Ghadyaan, Shaykh Saalih al-Fawzaan, Shaykh Bakr Abu Zayd.
Reproduced from Islam QA