What is the makor that if a nochri enters the sukkah the ushpizin run away and does this apply even to goyim who are shomrei sheva mitzvos bnei noach (or other nochrim today)?
There are people that are makpid on this. The mekor of this is from the Sifsei Cohen on chumash, parshas Emor pg. 174, since he succah is the “Tzela Dmhemnusa” and a nochri doesn’t have emuna. Also see Kaf Hachaim 639-6, However if not letting the nochri into the succah it will cause ill feelings it is permitted. See Melochim Umanayich pg. 345, Rivivos Efrayim 7-179. See Avnei Derech (Prince) 8 pg. 316.