Well fluxbox is a window manager, this means it JUST manages windows so yeah you need a login manager. Examples for window managers are gdm, kdm, xdm or slim.
If you just install (=emerge) fluxbox you'll get some stlyes, a rudimentary menu and no wallpaper picture. You have to setup everything else yourself but you can do (almost) everything you want...

If you want ideas of what you can do look at the the screenshot-section on the fluxbox webpage.