You must understand, hashtags isn't like subreddits on reddit.

They're more like Hashtags on twitter. When you post something, you choose a tag for it. This just means that the people who are going to see your link are: Those who follow you. Those who follow the tag you've chosen. And those who follow those who share your post. Tagging is for choosing what demographic of Hubski you think would find your post most interesting. So when you find something that is related to Firefly, you'd tag it as Firefly, this informes all that follow you that you're interested in Firefly, and if they agree with you in liking Firefly, they might just choose to follow the tag aswell. When it isn't there when you search for it, it's because noone has tagged something #Firefly. This isn't necessarily connected to this being a small community, but might as well be because other people interested in Firefly have simply chosen to use the tag #Sciencefiction, because it may resonate with more people, and it has a guarenteed amount of people that will see it.

You can always create the tag #Firefly if you wish to, by posting something and tagging it as such.