Questions about Buddypress

As I understand it, you can set up a multisite network, and offer have buddypress available, and have some sites that are not buddypress and some that are? Correct?

What if you have the main site IS using buddypress? That will not effect the other sites. Correct?

If the main site is set up as buddypress, it can be switched back to normal wordpress site? Correct?

I have not used this yet, and just want to be sure when and how it can be used.