The game is crashing and stuttering often, so it's been a pain to get there, but I finally hit 200 population. I had reached it a nearly a dozen times only to have the game crash before I could save it or while it was attempting to save. I thought I defeated alpha 3B at 199 pop.
I have ordinary plants filling up nearly every single square in populated areas to keep the morale up enough. I think I'm at 90 technicians and the oxygen recyclers appear stable at 78-99% quality.
Tips for high population game saves:
-Starting the game takes forever before people know what they're doing again, so you should pause the game for a minute or so. If you drag the map around and it still stutters, not everyone has activities assigned yet.
-If you play at fastest speed, keep an eye on people getting sucked around life support rooms. The middle speed appears the fastest you can go with stability.
If you want to play with the game save, it's here: https://dl.dropboxusercontent.com/u/2090222/spacebasedf9/bigbase1pop200.sav
It's 4.7MB, and your cpu will get hammered. Consider yourself warned.
Steam claims I'm at 64 hours of gameplay, and I think I've got about 30 hours into this one base. I did use the infinite matter bug to start out, but all the ordinary plants caused me to mine and demolish derelicts.