How much population should i have in relation to planetary size/production?

there is no perfect number, if the planet is attacked often youll want a bit and if you think the planet is about to be attacked youll want a bit. If not you should waste the space on non-planetary production buildings IMO.

I use to feel this way and mostly agree. NOW, I will spend a little space from time to time on cloaking since that is now more useful. If I have extra space I can't fill with production, I will fill it with population buildings.

If it's not in anyone elses start charts then just put cloaking on it so you can keep off other peoples star charts, if you just took it over recently and can't flux it, then max out it's defences so you can hold onto it long enough to get a planet fluxer. Pop for a planet is like hulls on your ship, and naturally defense is the same...Max out a ton of defense and fill it in with pop afterwards, pop w/o defense doesn't do you much good, but high levels can pretty much take any planet they want w/o too much of a problem.