why did Mr. Simon pair the two boys?

I think he saw potential in Nichol - the kid was smart - and wanted to give him a chance to show he really he did have an essential decency in him. I quite liked the film though I did think it went off on too many tangents: I would have liked more time on the Stanley/Nichol relationship, to flesh it out a bit more since it did seem like the main thread. And a bit more solid information on Stanley would have been good. Some of the structuring seemed a bit off, too: we got a postscript on some of the characters from the narrator while the film still had a while to go. And speaking of postscripts, the ones in the credit seemed a bit odd (if we assume they're from our narrator: he said he had lost contact with Stanley earlier. And I don't recall us getting an update about the black kid who seemed to be one of his best friends).