1.One should not make salaam when greeting a non Muslim. If a non Muslim greets one with salaam, then one should reply by merely saying “Wa Alaik” (and upon you), and if many non Muslims greet one with salaam, one should reply by saying “Wa Alaikum” (and upon you all).
عن أنس بن مالك أن رسول الله صلى الله عليه وسلم قال: إذا سلم عليكم أهل الكتاب فقولوا وعليكم (مسلم رقم 2163)
Hazrat Anas (Radhiyallahu Anhu) reports that Rasulullah (Sallallahu Alaihi Wasallam) said: When the Ahl-e-kitaab (Jews and Christians) make salaam to you, then reply by merely saying “Wa alaikum”.