Welcome to the forums.
Yes, CentOS 4.3 is a good choice (I use it
) and you do need the RHEL 4 Zimbra install. You should not use the Zimbra server (tomcat/apache/mysql) for anything other than Zimbra as they are tuned for Zimbra performance.
Vsftpd would be my choice for an FTP server but proFTP is OK if you prefer that. If you want to run apache/MySQL you can install those as well on the Zimbra server, make sure they use different ports. Check the forums for info on setting up additional servers. Providing your server meets the minimum specification especially memory, then you should have no problems with Zimbra.