Marco,
I use rsync to do both local and remote backup of multiple servers every day.
I usually create a public key with the server name its created to keep the from each other.
The I create a ssh config file in /root/.ssh/config indicating which key to use. Hence when connecting to diskserver01 use the archive01_dsa key etc.
Host diskserver01
IdentityFile ~/.ssh/archive01_dsa
Host server02
IdentityFile ~/.ssh/archive01_dsa
I only use the authorized_keys file and cat all the keys into that.
Rgds,
Jesper