Multisite bloggroups?

Hi!

I’m looking for a CMS for about a thousand different sites with some different needs – but with essentially the same basic ones. WordPress is quite close to what we need, but the thing is that some of these sites should have unique themes (only available for that specific blog/site) and should be ordered into groups (blog-categories if you like), so that we can appoint admins to maybe fifty blogs with just a couple of clicks.

So – the important things here are:
Can a multi-site installation of WP:

• Only allow specific site(s) to use a specific theme/plugin?
• Bulk together blogs so that we can quickly appoint admins for a part of the network in a simple way?

Preferably this should be done without having to hack too much and in an update-safe matter.

Thank you 🙂