I think the only way that you would get it as your default window manager is if you did it through the setup of your linux distro. If you add it in later, I don't think it will overwrite /etc/X11/xinit/xinitrc. Just use xwmconfig after you install the packages to make sure you are running the windows manager you want.
You probably just need the gnome libraries for dependancies... you can go to
www.linuxpackages.net and look for all the gnome libraries that you need, they even have a package containing all gnome1x libraries (gnome-libs1x) that is handy when you want to use gtk apps without having to have the full blown gnome. I am running fluxbox, but am running some gtk apps... here are the packages that I have installed to meet dependancy (for evolution 1.4.2):
gnome-keyring-0.2.1-i486-1
gnome-pilot-2.0.12-i686-1dl
gnome-pilot-conduits-2.0.12-i686-1dl
gnome-system-monitor-2.6.0-i486-1
gnome-utils-2.6.2-i486-1
gnome-vfs-2.6.1.1-i486-2
libgail-gnome-1.0.4-i486-1
libgnome-2.6.1.1-i486-2
libgnomecanvas-2.6.1.1-i486-1
libgnomeprint-2.6.1-i486-1
libgnomeprintui-2.6.1-i486-1
libgnomeui-2.6.1.1-i486-1
gconf-2.6.2-i486-1
Evolution works just fine with these. If you want full blown gnome, just install it off your slack disk... it shouldn't overwrite your x prefrences.