Nicknames are a form of friendly banter, and if a person prefers to be called something in particular, they could just ask
I don't fully know everyone, not even myself. So it is socially to call people by initials if you don't fully know them, for it is appropriate if you know them well but not completely.
Usually I would begin a conversation like" (Me=A) Hey B. Hey A. What is your name? I am Claire. -Now they could reply like- B. My name is Mathew, but you could call me Mat. or B. I am Mathew. A.Could I call you Mat? B. Yeh. So really, you don't have to know them well. Just smile, say hi, and after knowing their name usually you would get to know them more anyway.
Don't shorten my name. None of you know my name. But you don't shorten my name. If you shorten my name and your face is in tact then it means I like you a lot or cannot find a way to hit you and be the good guy. My name, which will not be guessed, will not be shortened.
Not if your just meeting them and especially not if its not a casual meeting. Its weird unless youve talked to them at least like three times