Answer by PLL for "Little Pussy Malanga, sometimes confused with fellow reputed mobster Big Pussy Bonpensiero, ..."— What does "fellow reputed mobster" mean?

Your first reading, which you rejected, is very nearly right. The confusion is just that “fellow” doesn’t necessarily mean “belonging to the same gang”, just “belonging to the same category or class”— it can sometimes carry connotations of shared affiliation, but it doesn’t necessarily. As described in Wiktionary:

(also attributively) A person or thing comparable in characteristics with another person or thing; especially, as belonging to the same class or group.

So “…fellow reputed mobster Bonpensiero…” means “…Bonpensiero, who (like Malanga) is a reputed mobster…”

James K’s answer goes further into the reasons why “…reputed…” is a common journalistic usage.