1. Only if it is your last planet - you can keep going as long as you have one planet left.
2. I usually go for the higher PQ planets first - but once you start to see a trickle of enemy colony ships headed your way, just grab anything you can, because there's usually a tidal wave right behind that first group.
3. No, the targeting sequence is calculated automatically during the battle. Why one ship is target first, I am not completely sure; I have a very holistic playing style. There have been a lot of extremely detailed posts over the past year on battle sequencing, though - try doing a search and you will find lots of results.
4. Most likely you don't have the right colonization tech. If you capture a radioactive world but don't have the colonization tech, you can't build anything there until you research or steal the tech. The other scenario is that your treasury is in the negative, but if that were the case, then all of your planets would be displaying Never.
5. There was such a thing in GalCiv 1; Terror Stars. Stardock is bringing them back, apparently, in the upcoming Twilight of the Arnor expansion.
Welcome to the forum, and good luck!
