Can a teacher greet his students with hallöchen? May I use this word to say hello to my German boss when he comes to our office? I see it’s informal but I would like to ask about the communicative strategy: what kind of people do we use this word for?
Generally speaking - no.
I would only use this greeting in very informal situations, and the minimum requirement would be that I say "Du" to the other person.
Nowadays, what is considered acceptable varies a lot with company culture - this also goes for universities/individual professors - so there is no hard and fast rule here. But generally speaking, you would have to be quite comfortable and informal with the other person to use it. Typically, this would be your friends.
If German is not your native language, I would suggest not to use the expression, unless your boss/professor has used it towards you first - getting those cultural connotations right is tricky even for Germans :)