How could I tell rsnapshot to keep the initial (ie., the very first backup) backup of my server forever, incase I may need it in some rare time (like for some files in /etc directory ) ?
There is a utility called rsnaptar which can be found in the utils folder located in /usr/share/doc/rsnapshot-*/ area depending on your rsnpashot installation. From the rsnaptar file itself is a brief description:
A quick hack of a shell script to tar up backup points from the rsnapshot snapshot root. Sends an e-mail to an address specified on the command line when finished.
So you can use this script utility to tar up your initial backup and keep it handy.