I always refer to crew I don't know as Captain or Sir, no problem being polite and offering a little respect. I'm in return always referred to by my first name or "Eng" and I don't have a problem with either of those.
As for the comments of all aircraft Engineers being Mechanics unless you've designed the bird, that's just nonsense. Shows a complete lack of understanding of what the job entails and the structure of the aircraft engineering world. Engineer is the correct title.