I don't know of any switchdesk command in Slackware but you can do it this way:
- change run level in /etc/inittab to 4 (graphical)
and you will have gnome login manager where you can choose your window manager.
If that doesn't work, change login manager in /etc/rc.d/rc.4 to KDE login manager (copy the KDE lines before GNOME) and that should work for 100% - you will have the option to choose the window manager.
Of course fluxbox has to be installed first
