Jeroen,
How can the unnecessary services be disabled? One Debian VPS is the source of SQL injection attacks and I have no idea how that happened.
Robert
for the purpose of running tor relays I would immediately want to change their distro to Debian.
Or easier: make a chroot with in that a Debian install.
Then you can run all Debian stuff easily, without having to bother with CentOS/RHEL/Fedora/etc...
Just make sure that you disable most services and/or keep these upgraded.
For a public relay you will want to do that anyway, as these hosts will be scanned by external "friendly" people.
Greets, Jeroen