Since ethos is used to describe the distinguishing character, sentiment, moral nature, or guiding beliefs of a person, group, or institution there can't be a plural..it encompasses the entire belief system, etc. not a single belief. A group can't have more than one distinguishing character, since distinguishing character encompasses all the individual bits that go to make up the distinguishing character. The only authoritative reference that i can give is that no dictionary I can find gives a plural or a plural use for the word.