Do I need to be concerned with importing existing GoG games into Galaxy (whether or not they were installed with the "galaxy-aware" newer installer, or the old school discrete installer)? I have a lot of mods and custom patches, such as hi-resolution patches, unofficial fan-made patches etc.

And yes, I did a search. Not super fond of the search algorithms here but I tried.117 page thread on Galaxy is a bit much to read through page by page, so my apologies for what is almost certainly a duplicate.

You can disable auto-updating, now what I don't know for sure if you can before importing the games. Maybe there are people who know more.

If you disable your intenet connection for the few minutes you are setting things up and until you have disabled the automatic updates, you should be fine. You are not very likely to forget to enable internet again.