Basically, I'm a bit concerned about having my good planets being taken over by other players, but I don't want to cut too much into production.
Any tips would be helpful.

depends what defences you've got.

Bad planets - pure production
Mediocre planets - production, no more than 8 space for defence
Best planets - I'd go up to 16 space for defences.

hmm, i would dispute that. If you have the best defensive buildings available and use buff buildings i can get away with a measly 8 space of defense. I have 6 space on everything for defense and my best. well, it depends really.

massive or larger planets = best defense
very large = medium defense if mining or arti... no defense if gas
large = 3 +10 production buildings (no defense)
average or smaller = no defense

i value size, not production rate