Did they call each other shortened names - Matthew Matt or did they have "middle" names like john "bear strangler" McGee?
Yes, they were called "agnomens"
"The agnomen (plural agnomina) formed an additional part of some Roman names, usually following the cognomen. Usually they were nicknames acquired at some point during the lifetime, but, rarely, some agnomina were inherited."
Examples:
Publius Cornelius Scipio Africanus
Publius is the praenomen
Cornelius is the nomen
Scipio is the cognomen
Africanus was the agnomen, given because of his victories in Africa
(Full name) Marcus Aurelius Marci f. Quinti n. tribu Galeria Antoninus Pius
Marcus Aurelius Antoninus is the praenomen, nomen, cognomen
Marci f. Quinti n. is the filiation. It stands for Marci filius Quinti nepos, meaning "son of Marcius, grandson of Quintius"
tribu Galeria indicates membership in the tribe of the Galeria
Pius was the agnomen
"Biggus Dickus" (Monty Python's Life of Brian) https://www.youtube.com/watch?v=2K8_jgiNqUc