All I can say is that you don't need to uninstall fluxbox, and I don't think that it would even help (not that it would cause any trouble, I don't think it would).
The arch linux wiki is quite extensive and handy (even for non-arch users, in regard to whatever is not specific to Arch), I bet it has the answer you need.
I guess the answer is here specifically:
PS.: I think that often the command "startkde" could work even without editing anything, but you may want to check more deeply into the possibilities of editing xinit or startx.