Stupid question possibly...

If you have a planet good for farming and has no defenses if you attack with a small force do you get all of the resources you can carry or only a percentage of the total?

I believe its a percentage. Not entirely sure. One of the older ogame vets might know. Good question though =)

I believe it is a percentage. You are only allowed to take up to 50% of a planet's resources, or something similar.

It is 50%. It also depends on your cargo capacity. Say you have a L Cargo ship which is 25 thousand capacity and your opponent has 50 thousand worth of resources you'll take 1/2 of his/her resources if you win the battle.