Nice plugin, but I have a couple of suggestions.
1. The Yourls custom fields are visible as you haven't added an underscore to hide them. This is most annoying as it clogs up the Custom Fields meta box, when all the options are already available in the Yourls meta box.
yourls_shorturl should be
yourls_tweeted should be
_yourls_tweeted to hide them from the custom fields meta box.
2. Is there any way of getting the Category/Tag hash to remove any spaces in the category/tag names, e.g. "some such tag" creates a hash tag "#some" followed by the words "such tag", instead of #somesuchtag, which would work.