•Do planets have a cap on how big a population can get before they stop growing? I do not want to get to have 2 Trillion souls on a world as that seems a bit too much.
No caps, but growth slows so much you will never get to 2 trillion without SMing in population.
•If for example I set a planet to have 50Mil. Pop. limit with infrastructure on worlds not yet terraformed(Thus the 50 Mil. won't grow due to infrastructure limits) does the infrastructure keep coming in from commercial ships? 'Cause it seemed so from my game. Is there a way to stop this?
No. Also seting a specific limit with infrastructure dosn't work as A) The colony builds it own very slowly without using material, B) there isn't a good reason to keep a population from growing, and C) it doesn't stop people from growing, they just start dying en-mass without enough infrastructure.
•Can I setup a PDC with a very large PDV so that the pop will never ever complain? Or is it better to have a very large station with military weapons in the system close to the colonies(I decided to play a new game without overhauls since I reaaaally like forward bases with hangars and ports and such cause it's cool that's why). This is also tied to the first question, as I really do not enjoy have Trillions upon Trillions of people that just need more and more defenses. There should be a limit on this no? Otherwise it just gets insane.
Larger populations and more important planets it is better to build one large station. For multiple smaller colonies its better to build small outposts. Also, I simply don't see a rational or logical reason for there to be a cap on PPV requirements. A larger population feels like a larger target, and therefore want/need more protection. If you really wanted to skimp on PPV, you can always train large amounts of cheap ground units to act as "police" forces.