Creating a secure base

> Is it possible to create a base in a workspace but only allow certain members of the workspace to view it?
I'm afraid not.  You'll need to make sure they're not invited to your workspace and invite them to the individual bases you want them to have access to

> Same concept, but within a base...how can I create a public base but lock one of the tabs to be visible to only a few members of my workspace?
Hmm, if it's visibility only, then you could create a shared view with a password?  If you need to be able to edit, I don't think this is possible either unfortunately