PLESK (and webmin) is great for managing CentOS - a full distribution of CentOS, that is. There is no point using it for SME Server, which has been paired down to a bare essentials appliance and already has a set of admin screens for managing what is already there.
Plesk and SME Server administration functions will also clash, since Plesk changes config files directly, and SME Server changes the config files via a templating system.
-- Jason