One of the interesting points that I've read on extreme planets, is that no matter what, the AI will spend a large amount of their time researching the technologies to take at least one type of the 5 exotic planets at the beginning of the colony rush.
The benefit here is that they leave planetary invasion until later on in the game.
I can't remember which topics they were, there has been a fair amount of discussion on this so they should be easy to find, but in a nut shell if you dispense with trying to colonise those exotic planets in favour of gunning for planetary invasion you can gobble up many of the other races standard planets before they even have the means to retake them. Really I suggest you forget about the exotic planets until later on in the game.
just something to keep in mind, but on the second issue you're having it does sound like you're over extending your self in the mad rush for planets, Try to gobble up the planets that are PQ10+ first, and if your economy can support another then move onto the PQ9 and under.
And if those darn aliens take a nice lil PQ10+ Exotic planet in the area you wish to control, you can take it by force.
