I want to add a sub-category to the 6 java speed run page but am not sure how to go about adding it. I want to add Any% Set Seed Co-op.

Only game moderators can edit categories for a game. Minecraft java edition has a category suggestions channel in the discord server, so you might want to head there to ask for it to be added.

If they don’t add it then you could put it in this doc:

https://www.speedrun.com/mcce/thread/rnyd4

thanks