"NEVER MIX BINARIES FROM DIFFERENT VERSIONS (OF SLACKWARE)". *Most* of the time this rule of thumb is simply not true. You may have to carry forward some libraries, but that is all there is to it.
Actually the last KDE 3 (3.5.10) was for 12.2, and I have been carrying forward that version for quite some time. I think that missing hal will be the stumbling block for running that version for 14.0.
At the moment I run Trinity 3.5.13 on Current (which at the moment is identical to 14.0). There are some glitches, but nothing fatal.