How to install PostgreSQL to run under a different system user than "postgres"?
Solution 1:
Stop the database server, change the ownership on the Postgres data directory, update your startup scripts to reference the correct user, and start the database server again.
Document your changes so someone expecting Postgres to be running as a traditional Postgres user account won't be unpleasantly surprised.
If you are using RedHat's packaged Postgres installation (installed from yum/RPMs) you will need to be aware of this change and make sure to check the startup script when you upgrade to ensure it hasn't been overwritten.