I guess some topics like how much they earn can be considered unpolite to talk about, but in general just try to be ready to drop a topic or question if they say they are not open to talk about it or that it's personal, without making a big deal about it, and you should be fine.
In general, if you try to be a good person, that's enough. People come and go, so always look for new ones to replace who's busy or leaves.