Also, how could all KDE packages be removed easily afterwards without breaking something that I need?
I would take a look at /etc/rc.conf for what concerns the boot-up daemons (slim in place of kdm, for instance)
This could be really hard to achieve, depending on what are your needs (for example, it will be difficult to remove every KDE file if you use Konqueror, Kate, Konversation and K3b): anyway you could be interested in this REMOVE option:
as well as this SYNC option
Hope that helps. :D