Well for the Network Edition you will need to use a supported operating system if you want to take advantage of the support provided by the Network Edition.
I would personally recommend RHEL 4, you can get a basic subscription for $300 and a small price to pay to ensure you have the support for your email services that you need.
While CentOS is binary compatible, I do not believe that Zimbra support would be extended for it when running it in place of RHEL.
EDIT: Welcome to the forums!
