Most of your questions are answered in the wiki, you should read it
There is a guide, courtesy of wolfden, in our wiki, check it out:http://wiki.sabayonlinux.org/index.php?title=Visual_Tour:_CoreCD
it is targeted toward advanced users.
Gentoo documentation is perfectly compatible with Sabayon.
Sabayon has an additional overlay with custom or additional ebuilds, but Gentoo ebuilds can
be used with it.
Sabayon can use a binary package manager, entropy, which ease the package management and is
compatible with portage; mixing them can be done, provided that the user knows what he/she does.
Its TUI is equo and its GUI is Sulfur, the CoreCD version is able to use both entropy and portage.
Hope it helps