You might also want to adjust /etc/hosts after you have added new IP addresses, although this is not necessary.
SELinux is a security extension of CentOS that should provide extended security. In my opinion you don't need it to configure a secure system, and it usually causes more problems than advantages (think of it after you have done a week of trouble-shooting because some service wasn't working as expected, and then you find out that everything was ok, only SELinux was causing the problem). Therefore I disable it, too (this is a must if you want to install ISPConfig later on).
If you are following this tutorial to the tee like you should, be advised a newer kernel update is available and will be upgraded and will affect the compiling of courier etc, you will not see a problem until after a subsequent reboot. I would either recommend disabling kernel updates before doing the yum update in step 7 Install Some Software or rebooting after the yum update in step 7 Install Some Software and then disabling kernel updates. Took me quite a while to figure out this problem as I am a newbie to Linux. I hope this helps out other people.