I know that one requirement to use the KDE apps would be that you have the KDE libraries installed - you can use the Slackware "pkgtool" utility to go in and install those. I'm pretty sure there's several other libraries that need to be installed as well. I use Gnome as my window manager, and most of the core libraries were already installed when I installed Gnome (GTK libraries? I'm sure someone will know
) - FWIW, I use Quanta+ 3.3 with no problems in Gnome.