It will work normally. You can call any KDE or gnome application from, for example, a terminal and Fluxbox will open it. What can happen is, in the case you want to install an application, some files on your system might be missing, like kde-base or qt or gtk or whatever. Well, if it ever happens, just install the missing package(not the desktop)
. I use some gnome applications on my Blackbox, as gnome-run without having gnome installed. So yeah, it works
.