Can I use rhel packages on centos? Specifially I am concerned about this repository: http://yum.pgrpms.org/9.1/redhat/rhel-6.3-x86_64/
CentOS is basically recompiled RHEL packages and you can easily install RHEL package on CentOS host. The version number and architecture have to match, of course.
BTW, for postgres we use packages from http://yum.postgresql.org/ on Scientific Linux (yet another RHEL clone).