Multisite newbie questions

Hi,

I have never used WP for multisite, but now I feel like I have to because one of my projects. I have few quick questions which I would be very grateful if you could answer me.

1, Can I use different theme for each site?
2, I believe I can have “network” site wich connects all of the “subsites”. Is it possible to display all articles etc. from all of subsites?
3, I need to collect some additional information about my users, is there any plugin for multisite version, which allows me to add some inputs to “user administration”, and also allows me to filter users by this information + allows me to export user list?