I think you fundamentally misunderstand the Discourse permissions model. Every new category is always visible to all users, unless you set up a group and lock down the security on the category in the Edit Category panel. It sounds like you expect each new category to be visible to nobody after it is created?
Discourse is “public by default unless you tell the software otherwise” so statements like
.. make no sense to me, the public areas are public, that is not a “leak” that is normal.
If these groups of people have literally nothing in common, then no, I actually wouldn’t shove them all in the same house..