Weird displays of power

As I read through The Silmarillion, and the other works appended to the beginning (Ainulindale, Valaquenta in my publication) the explanation of power seems confusing, if not inconsistent.
For example, in the Ainulindale, it says that Aule has almost as much power and skill as Melkor, but in the Valaquenta, he is third on the list of Valar, and that list is ever after adhered to. Furthermore, if Melkor is more powerful than the Manwe, the most powerful Vala, why is he scared to death about Tulkas? And what about Ungoliant? How in the world is Ungoliant so much more powerful than Melkor? Even if she has absorbed power, how does she even have the potential to compete with Melkor? I supposed much can be explained by Melkor's power residing in those whom he has perverted, which is stated in the Silmarillion, like everything, later than necessary, but it still does not explain the whole Tulkas thing. Why did Tulkas even come to help, since every time he encounters Melkor he simply pins him easily, when Orome is said to be almost as strong as Tulkas? I just don't understand.