To set up different categories you might want to investigate
Video nodes (wiki) but there is no reason your shorts will not show up in the general 'movie' category, unless as 'nickr' suggests, the scraper database just doesn't have a listing yet (and yes you can add to the meta-data of the database site ,then everyone befits) or create your own
Custom video entries (wiki) primarily used for home videos.
I've run into this issue myself, and have handled these listing in various ways, from file mode folders through to custom nodes
Add-on:Super Favourites (wiki). I suspect if the short is a commercial variety, you might find the listing in one of the many meta-database sites, but you might have to look beyond simple scrapers (look for a browser search to turn up a tt# that might be inserted directly into the Kodi scraper) or end up plugging into a custom .nfo Keeping this issue alive in the forums, effectively brings this issue to the attention of those who could help.
As a long time user, I'm sure you are aware that their are 'millions' of Kodi users, and a few bucks from one user isn't going to do it. The project is enormous, millions of lines of code, many developers, and a huge team effort working feverishly to bring Kodi to the fore, there's going to be shortcomings as new equipment comes to market, road map diversions and bug stomping. The overall health of the project requires that we all do our part, which also includes regular donations to facilitate development meetings to help resolve some of the most pressing issues users have. Not the least would be the future of this open source free software.
Let's look at this from a slightly different perspective, if this was commercial software you would have had to put a price tag of $100+ not to mention the updates and iterations commercial products charge. To keep this product viable, we need everyone's support, and yes this is a beg to turn your name into the tag 'Doner'.