Comment # 2 on bug 998919 from Stefan Seyfried

Unfortunately I see no easy way to only start nm-applet if NetworkManager is
running.

The choice of wicked vs NetworkManager is done by YaST depending on the
Hardware type, and has nothing to do with XFCE.

nm-applet could just not show the icon if NetworkManager is not running, but
that's something you would need to issue upstream at
https://wiki.gnome.org/Projects/NetworkManager

So I'm closing this as "wontfix", even though it really is more like a
"cantfix" :-)