I did some pinning so that I could run KDE 3.3, but my main system is testing. This won't make it too unstable, will it? I use it for mostly desktop and some production use.

Also, when I apt-get remove and install kde, why does it say it is version 5:42 instead of 3.3?

Thanks

It works just fine, just set the pinpriority for unstable below 100. This way unstable packages will only be upgraded manually. It won't upgrade automatically which could lead to the installation of more unstable packages or dependencies conflicts with sarge packages.

Use apt-show-versions -u to inspect possible upgrades and run apt-get with the -s switch to test before an upgrade.

Upgrading all unstable packages at once with:
apt-get install `apt-show-versions -u -b | grep unstable`

You don't have to remove kde in order to upgrade to unstable. Even so I wouldn't use the metapackage, but specify the real packages.