It wouldn't take that much space if you'd just reduce the trailer size by a third. But i'll take what you will do even if i don't like it.
Also, it's justa line, so i don't know why it's doing what it is doing on the guest page either. It looks like you thought of it in a very difficult way instead of just adding a simple line of text in between each category...
Also, on the front page only, could the "category" of guest be added as a tag on the same line.
So:
William Shatner (Special Guest Star)
Croatoan
Instead of:
Special Guest Stars
William Shatner
Croatoan
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Essentially, though, even if you go with the latter, you shouldn't have to add more than one line. Just change Guest Cast to Guest Stars and then allow the option to add Special Guest Stars and Costars. The vast majority of episodes don't have SGS and Costars, or there are eight or more guest stars. Which means that no costars would be listed on the main episode page because only eight stars list on the main page. If you prioritize it so that guest stars and SGSs come up first, costars will relatively rarely appear on the main episode page and thus no second header.