Adding these lines to the bottom of your xinitrc file does not work because the line that runs xfce runs in the foreground, and therefore the commands you added will not run until after
the xfce process ends. Add these commands before
the xfce line, and follow the new lines with an ampersand. For example, my fluxbox xinitrc file reads (at the bottom):
xscreensaver -no-splash &
gkrellm -w &
The ampersands cause the processes to run in the background, therefore the next command is executed before the previous processes are ended. Your window manager actually comes up last, but the applications you invoke in the background will be displayed.
Hope that helps.