I am in need of a single word for "a chieftain of thieves", if there is any. If there is none, can I use "thieves'-chieftain" instead?
Capo sense #2
The head of a crime syndicate, especially the Mafia, or a branch of one.
One possible alternative is kingpin. Wikipedia gives some other suggestions.
A crime boss, crime lord, mob boss, kingpin, or Don is a person in charge of a criminal organization.
Guildmaster would be a good term for fantasy or science fiction or medieval historical fiction. This comes from the archetypal "thieves' guild" that's prevalent in Dungeons and Dragons and all of it's roll-playing variations over the last 40 years. Many best-selling fantasy authors (R.A. Salvatore, for instance) have used the expression in their works.