It has always been possible to install and use KDE applications in Gnome, or vice versa, as long as the supporting libraries are there (and generally apt handles that for you). But getting them to look right is another story. More here

Starting with Hardy, KDE4 became available in Kubuntu, and by the time of the Intrepid Ibex release, KDE3.x was gone. It was no longer possible to install just the systemsettings tool to control font face, font size, and LCD subpixel hinting; doing it that way left those choices missing from System Settings.