Slackware includes Amarok and libgpod, and they should be installed by default. I would suggest updating your machine to 12.1 if at all possible though, as you need the latest version of libgpod to support the Classic and new Nanos (and I am not sure which version shipped with 12.0). Album cover support and all of that is all handled by Amarok. As long as your ID3 tags are good, there is nothing to worry about.
You do need to set the FireWire ID for the new style iPods. This is a relatively simple matter, and only takes a minute or two. Once done, you never have to mess with it again. This section
of the libgpod Wiki explains what needs to be done.