Fag is a derogatory term for a gay person. Or a term used for insults in general. And rats are commonly associated with dirty places. So when calling someone a "Fag rat" you are calling them a dirty/ gross "fag". Also used as an insult in general nkt specifically a dirty person
City Rat is a term for a person who are usually young teenagers who hang around the metropolitan area for no reason other than to cause trouble. They can be usually found on trains and alleyways. Also usually rocking their Nike TNS and Adidas shoulder bags. Often more than not, city rats will get defensibe when called a "city rat".
That Shelia, Sofia is a city rat just like her friend saba.