Is there some sort of template available to fill in the details of a spell, a monster, or an NPC for the benefit of writers like myself?
Sounds like it would be easy to make one, but I'd like to know what needs to be included in all such Pathfinder-compatible elements.
Normally, if you are writing for a publisher, they would provide you their preferred format so you can write it in the way they want it.
The formats aren't 100% the same between companies; Wizards had a format for its stat blocks, the Game Mechanics had another, and so on. I don't know if other companies that are publishing Pathfinder materials are using Paizo's format or if they're making their own, but if you're writing for them, they should be able to provide a style document for you. :)