Won't the aliens just blow up the new planet again?
According to the tie-in novel, the Drej came from another galaxy, and those who were destroyed/absorbed at the end of the movie were the only ones in our galaxy.
Even worse, considering that the Titan was activated without any thought to astronomical placement of the new planet, it seems highly unlikely that it will survive any length of time. For example, New Earth was created in the middle of an ice field. This might seem to imply that the new planet isn't in the correct orbit for a habitable planet, but that fails to take into account the effect greenhouse gases have on Earth's atmosphere. Without greenhouse gases the planet's actual temperature would be around -18 degrees Celsius, therefore the correct orbit does not mean instant liquid water.
Considering that Cale's father specifically picked the ice field to hide the Titan in, however, it isn't too much of a stretch that he picked that location for other reasons as well. This troper always assumed that the ice field was chosen specifically to provide the Titan with the raw materials TO create a planet, and that being a good hiding spot was just a coincidental bonus. For example: funny how the ice melts into just the right amount of water for a Class-M planet, and provides enough cooling to allow "Planet Bob" to be habitable just after being imploded into existence..? Hello? Genius scientist, anyone?
After a close watching at the end, I saw that there apparently is a sun nearby the new planet. It must have been out of shot for a while. As for the stable orbit, thats a bit more tricky. Then again, this is the 31st Century and a spaceship just built a new planet from scratch. They probably have ways of rectifying this problem.
Again according to the novel, the core of the Drej mothership is a yellow-white dwarf star. It is implied that once the ship goes poof, this star becomes the new primary for Planet Bob's system.
Why does Korso believe in the Drej paying him for his help? As far as it is shown the Drej live all by themselves and need nothing additionaly, so why would they even have any currency? And then there is the question how Korso doesn't seem to plan in that they want to kill all humans and he himself is a human. After him recieving the payment there would be no reason to not kill him, too.
The "payment" may be leaving him alive as the last human. Since - save for cloning - he couldn't reproduce, he wouldn't be a threat, or at least not a big one. Even cloning would run into a dead end due to the small genepool. They could also just take the payment from a third party by force and give it to Korso.