Are there strict guidelines about when you can include major characters in your stories? I'm thinking of when Batman and Catman have that awesome confrontation on the rooftops of Gotham. How do you go about including such a major character in your stories?

Oh, yeah, you can’t just drop Batman in your script without getting it all approved.

It’s totally fair, Batman is in a lot of books, they want to make sure his appearances elsewhere are effective and don’t mess with the main books. 

The big companies are not arbitrarily trying to make if less fun for writers, it’s just common sense to try to keep an eye on what your flagship characters are doing.