There aren't fixed rules for pluralizing words ended with "ão". (it can be "ãos", "ães" or "ões", depending on the origin of the word).
- MÃO > mãos
- aleMÃO > alemães
- liMÃO > limões
- cidadÃO > cidadãos
Please, refer to this article for more information: http://brasilescola.uol.com.br/gramatica/o-plural-das-palavras-terminadas-ao.htm
Well, "citizen(s)" and "city-dweller(s)" can be synonyms, but they are not the same word and can have different meanings. The first is "cidadão(s)" and the second is "morador(es) [da cidade]". (Likewise, another synonym is "inhabitant", but it is "habitante".) However, one can be a "morador", but not a "cidadão". Not sure if it's same in English, but if I say "Ele é um morador, mas não um cidadão" [He is a city-dweller, but not a citizen", it implies he lives in the city but has not the citizenship status i.e. is an illegal inhabitant or metaphorically someone without citizen's rights.
Citizen and city-dweller are quite different in English I know. When you say "can be synonyms," do you mean they might have the same referent or that they're really synonyms? My idea was speculation based on French, in which the cognate "citoyen" CAN, but generally doesn't (80-90% of the time) have anything to do with citizenship. (The Cambridge dictionary does give "inhabitant of a city or town.")