my friend… i just never heard anyone use a.s for caliphs. I might be wrong here and unlike u i will answer the question or admit am wrong when i am wrong. So dont worry it was something new for me, which did surprise me. I will do my research and will admit am wrong if i didnt find anything to back myself up :).
Okay so …
“ASAlaihi Salaam - peace be upon him.
Alaiha Salaam - peace be upon her.
Alaihum Salaam - peace be upon them.
This should always be said whenever mentioning any of the Prophets from Adam through to Jesus, or any of the women regarded as being the most notable women created (namely Eve, wife of Adam - Hajar, wife of Abraham - Asiya, one of Pharaoh’s wives who joined Moses on his exodus - Mary, mother of Jesus - Khadija, Muhammad’s first wife - and Fatima, Muhammad’s youngest daughter).”
http://www.bahagia.btinternet.co.uk/glossary.htm**
sunni
" A.S.** refers to 'Alayhis-salaam, (God’s) peace be with him. It is said after the names of all previous prophets, their mothers (e.g. Bibi Maryam [Mary] A.S.), the twelve divine Imams from the household of Prophet Muhammad (S.A.W.) and Bib i Fatima (A.S.). It will change to 'Alayhas-salaam (peace be with her) if it follows a lady’s name. After any two names we say 'Alayhimas-salaam and after more than two names or when referring a group of people we say 'Alayhimus-salaam. Thus, we sa y Imams Hasan and Husayn (A.S.='alayhimas-salaam) and the Ahul-Bayt (A.S.'alayhimus-salaam). "
http://www.al-islam.org/about/resources/glossary.html
shia
well glossary is the best i cud do, i didnt find the related topics anywhere else. I dont have a hadith or a verse from Quran so if u can bring me something better than just glossary from Islamic sites then i wont have a choice but to agree with you. Also if u cud find a.s used for any of the fist three caliphs anywhere by any scholar that wud be a bit convincing too.
I have tried to look for something in ahmediya site but cudnt find glossary there, However i didnt see a.s used for any of the caliphs in ur site.