There are no rules. There are only techniques that work, and techniques that don't work.
However, that said. the chances that you really need to stop the narrative dead to give the protagonist's entire history in chapter two (or, in the case of short fiction, starting at the top of page three) are slim.
Nobody wants to read that. It breaks the flow of the narrative, removes the tension and enjoyment the reader would otherwise get from learning about the characters in a natural and evolving fashion, and generally, well, doesn't work. Books are like icebergs; the writer should know 90% more about the characters and world than makes it on the page.
And I can say this, because I just handed in a book with a chapter two flashback.
And a prologue.
Although I cheated and called the prologue chapter one, and the chapter two flashback chaper three.
Did I get away with it?
Well, tell you what. This time next year, you can tell me.
mrissa sez: elisem shared something Mike once told her about unfinished projects being nurse logs in the artist's forest ecosystem, and that was such a kind, useful, and true thing to hear.
And some other smart things.
And it's true. You don't always have to finish something for it to be useful.
Valuable lesson for the compulsive completing things Bear.