Of course, but minimum word counts for bigbangs tend to direct them more towards lengthier pieces than short stories. Of course someone could do short stories as part of a mixed media piece, but then to refer back to the requirements I've sorted out for that, my submission will be a collection based around a theme and the same characters being featured in different mediums. This is obviously really subjective and variable, but if I was doing original pieces then there isn't the backstory context of using a transformative character (therefore a collection of small multi-media pieces have the benefit of referencing back to familiar canon character, rather than trying to keep an audience connected with a character with less opportunity to keep that character familiar without the additional context), it could potentially be a bit trickier to keep a unifying feel to the submission.

Actually I think 15k of one coherent fan story is pretty much equal to 15k of one coherent original story, but this is a multi-media bang, so it's not going to be that simple and it's probably better identify and address potential issues beforehand.

And hey, for the record, I think it's good that you're campaigning for the opportunity to express your choice of creativity. I usually sadface when I see a great bigbang that only allows art, not mixes, to complement the writing, so I totally get where you're coming from. All my quibbles here are very much 'maybe think and evaluate this' not 'don't do because' things.