I'm assuming you don't have the game yet from your questions.. You interact with planets through a development set of things you can build such as better Emergency Infrastructure (withstand bombardment better), Better Logistics, Better Tactical, and Better Population Infrastructure. That last one you can research to allow for higher planet populations. The planet populations grow slowly on their own, and different planet types (Terran, Volcanic, Ice, Desert etc) have different max population caps which can also be affected by special attributes you find on your planets by purchasing the "Explore Planet" option.
Pirates are fairly effective to annoy the AI or other players, you just up the bounty on them to be the max above yours and everyone else's and that player will get attacked by the pirates next. I don't think they are too powerful and have never had any issues defending against them. Just watch out though, the AI will buy them onto you and then attack you simultaneously from another direction with their fleet, real sneaky like!
Set build order, not necessarily. I have seen lots of people have success with different types of fleet make-ups and focusing on research or economy vs. rushing early game etc. There is definitely a balance to be had between all 3 aspects and how you build your empire.
