In case you have not seen it, have a look at How to use category security settings to control access to content.
You only need to create a group for your paid customers to accomplish this. For all categories other than the two categories that you want users evaluating your be able to see, add a security rule something like this:
Replace the “Shareholders” group in the screenshot with the name of your paid customers group.
With this setting in place, users who are not a member of your paid customers group will only see the categories that you have not applied this security rule to.