Najmi,
I think you might need to recheck the versions you are quoting.
With respect to RedHat, there was no 2.5, 2.7
https://en.wikipedia.org/wiki/Red_Hat_Enterprise_Linux#Version_history Secondly the supported platform document for our older versions can be found by clicking on the Archived Releases section of the product and documentation download pages. These show that 2.21 was supported on RHEL 3,4, and 5, there is no mention of a RHEL 2.5 or 2.7
With regard to your second question on Centos, for a server to appear in our supported systems document that means we have it in our labs, can build ports, and run QA tests against it. We have a RedHat Server, we don't necessarily have a Centos server. Centos is built from the same sources as RedHat so the expectation is that if something runs on RedHat, it will run on Centos. In the very very very unlikely, some would say impossible event that our product behaves differently on Centos than it does on RedHat, it wouldn't be upto us to change our product, it will be upto you to convince Centos that their product is not compatible with RedHat. A number of sites use Centos quite happily without issue.
Reuben