Is it best to stick with a single genre when you decide the type of story you want to write or can mash them up. Example :- Romantic/western, or Comedy/horror, or Si-fi/historical/Crime.
Some seem that they could go together and some are just a non-starter. And if you have a mixed Genre which section do you list it as when you come to selling and promoting your book. Please let me know your thoughts. Thank you.
Damien said it all... and well said. ;-)
Dor said it all... and well said.
My thoughts are: write the book.
A genre classification is there to give us a broad idea of what kind of book it is. Some, like Romance, have very specific rules about what can happen in them (eg: a happy ending). If you wrote a Romance which didn't have a happy ending, you wouldn't classify it as Romance (unless you *wanted* a lot of 1 star reviews).
Genre is (largely) a marketing issue, so think about who your reader is and how best to sell your book to them. Ben Aaranovich's books are Urban Fantasy rather than Crime, for instance, because he's got more in common with Neil Gaiman than Val McDermid.
More generally, I think anything can go together. a Sci-Fi/Historical for instance would probably be classified as Steampunk.