I'm assuming you have some version of RH. I don't know who you talked to, but as far as KDE or gnome and webmin, they can be on the server. The question is if you want the KDE/gnome/GUI on the server. In general, the GUI will just take system resources and is not *absolutely* needed for proper server operation. Samba should not interfere either, but I've never needed it on a hosting server.
Note about Webmin: you can use it for doing some tasks, but not all of it's features will be compatible with the way Plesk does it's virtual hosting and other things. For example, webmin's Spamassassin module eventhough you can make changes, those changes will not do any good with the PSA-SA. You could make changes to the DNS zone files, but the next time Plesk makes changes to a domain, the named.conf and zone file will be overwritten. The standard Webmin-Qmail module (limited as it is) will allow you to change some of the control options, but you will not be able to view the Qmail user's mail due to the maildir structure.
If you do use Webmin, make sure you go through each module's config options and make sure they are right for your system, especially the start/stop command options.