Ok, I tested this myself. I gathered up enough cash and materials to blitz the system; six colonies set up as fast as I could fly between the planets (I figured if I was going to sit and babysit I would get the most value out of the time. . . ). I decided to forgo any econ development until I had a system that could take care of itself, so my build order for each colony was patrol HQ>High Tech Station>Ground Batteries. I upgraded the Patrol HQ on 5 of the 6 planets first, so that was their first industry.
The Independents did send a (single) bombing expedition, but that arrived *after* I had my first two HQs upgraded to "military base" (they finished 10 days before they were scheduled to arrive in system). I camped right next to the targeted colony and never saw the expedition; it got jumped as soon as it got there and was wiped out (-5 rep, nothing I cannot make up later).
There have been no other expeditions yet, and my colonies are now nearing size 5. I am now working on econ to make them super profitable.
It is my understanding is that is the cutoff for generating bombing fleets, but even if it is not they are more than capable of handling themselves now, what with having high commands and battlestations (soon to be star fortresses as soon as I can scrape together the cash, building six colonies at ones if a bit of a drain on resources as they all wind up needing about the same things at about the same time).
I am off gathering AI cores to help things along, and I will not be rushing back even if the system is targeted.
Thanks everyone for the help, I felt confident enough in this course of action I did not even make a backup of the save file to test with.