I know that for this category the player has to save all 5 main characters. However, I know that it's possible to save Junior, one of the characters that work for Olson. Do I have to save Junior to get a run approved for this category? Or can I just save the 5 main characters?

I think saving the 5 main characters is enough.

yes, you only need to save the main characters

Wouldn't it make sense to make them subcategories for Solo? Theatrical Cut and Curator's Cut?