What do you think of my theory that Saul was a cartel lawyer and that's why he knew how to advise Walt ?
I think it's possible, and definitely makes a lot of sense, but you got one thing wrong.
His name isn't Miguel Goodman, it's Saul McGill. He's Irish.
Season 2, Episode 8 "Better Call Saul":
Saul: Mayhew. Is that Irish or English?
Walt: Irish.
Saul: Faith and begorrah! A fellow potato eater! My real name's McGill. The Jew thing I just do for the homeboys. They all want a pipe-hitting member of the tribe, so to speak.
Fred.