1. For all intensive purposes gentoo bases systems canot use deb files.
1.a. To install "amsn, frostwire, wine" there are two options, a) Spritz (a nice GUI http://wiki.sabayonlinux.org/index.php?title=En:Spritz
or b) a command line tool called equo (like apt-get) http://wiki.sabayonlinux.org/index.php?title=En:Equo
2. Its included on the DVD version already, I wouldnt reccomend trying to install it on the CD version as newbie.
3. Yes, you just need to add your username to the sudoers file, heres some nice ubuntu docs on it https://help.ubuntu.com/community/Sudoers
4. Cant really say, the user makes each distro work or not.