I have a copy of Susan Cooper's The Dark is Rising in a 5-in-1 hardback. I posted it to my shelf, then looked at the details of the listing. My copy has a different publisher and is listed at ~100 more pages.

It was originally published as 5 smaller books, the second one titled The Dark is Rising. I know the rules say the things that must match are ISBN, author, title and cover type (hardcover, PB, etc). What about publisher and number of pages - is that a deal breaker if they don't match?

Publisher and page #s don't have to match.  The publisher info can seem to be different when there's an "imprint" involved -- a smaller division of a larger publishing house, and the imprint name is used in one place and the larger publisher used in another.  They look different, but both are technically accurate.

For example, Pocket Books are an imprint of Simon and Schuster; Viking Press is an imprint of Penguin Books. 

So you might see "published by Viking Press" on your book, and the listing could say "Penguin Books", and they're both technically accurate.

Do double check that you're using the correct ISBN -- it isn't unusual for ISBNs for more than 1 edition to be listed on the verso (back side) of the title page. 

Also sometimes when they reissue books under the same ISBN they might add book club notes, upcoming book excerpts, author interviews etc.., or just changing the font and type can adjust the page amounts. 

It's up to the requestor to have an RC if they want a specific edition of that ISBN.