Webcomic fits perfectly well in 'medium'. Solicits, on the other hand, you're right - like most everything under the 'in-joke' group, I can't come up with a good class for it.
IMO, the most useful tag classes are char, creator, title, publisher, genre, and admin, and medium is defensible under the 'consistency' doctrine - although, could perhaps be renamed - technically speaking manga is the same medium as western comics, for instance - perhaps 'format' (possible tags within that class: webcomic, manga, manhua, Euro, American).
Improvements, to my mind: In-joke should be lost, or replaced with a less problematic class, and the 'char' class needs to have the tag format revised.