changing theme based on page and page level control

I would like to know if it’s possible to do a couple of things.

#1) Can I have control over pages? – some should be displayed for the public, and some not.

#2) I want to do three blogs in one. Each blog I would like to have a different theme. Do I need to install WP MU, should I install multiple WP, or can I do this with the standard WP?

I want to have a page that only clients can access, the people who work for me would access and be able to add to it, and a separate webpage that the public can see and gets listed as per norm in search engines. Each client would have their own blog, but not one they can add to or modify. An artist does work for a client, so only the client can see it, and only the artist upload the images. I would like a general webpage for the company. Finally, I’d like a personal weblog about the places I visit. I would prefer to do all this with my current installation, but I’m unsure if it’s possible. Any information would be appreciated.

Cheers.