Can we be private and public at the same time?

Yes, each category has a security tab within its settings so you could set one category to be viewable by “everyone” and restrict all other categories to “trust_level 0” — which is a group all registered users belong to.

We have a video that explains how to setup permissions:

https://meta.discourse.org/t/video-how-to-create-a-private-category-using-group-permissions/136439