So, there are comms out there for Buffy/Sam, Faith/Dean, Cordy/Dean, Buffy/Dean, and any/all BtVS/SPN crossovers. And several of us suddenly found that we were shipping Buffy Summers/John Winchester pretty hard (hey, she likes 'em tortured, broody, and older, he likes 'em blonde and fighting evil; there have been pairings based on less). Et voila, buffy_john.
Fic should include title, rating (any system, just so long as we can make sense of it), and disclaimer, and should go behind a cut tag. The Buffy/John pairing should be a major part of the story, though polyfic (Buffy/John/Other, an open relationship, etc.) is totally acceptable. What does "major" mean? Decide for yourself, but it should be more than a throwaway mention of eight words in the middle of your 2,000-word Faith/Dean.
Images bigger than 100x300 should go behind a cut tag. If you've got icons, three samples before the cut are fine.
Keep posts on-topic. If you have to ask whether it's on-topic, it's probably not, and if you really really think this is a valid exception, talk to the mod.