[Suicide Junkie]

#2:
Cargo requirements are usually quite easy to estimate. Round your small/medium sats up to 100kt.
Fighter counts can be rounded off to the nearest 5 or 10, and worked out from there.
Putting a cargo base in orbit can help. Any excess cargo the planet can't hold will go into the base.

#4:
Just build weapon platforms that are all shield generators.
That will give your planet a few thousand hitpoints that works just like armor.