Emile,

DKIM is used for both inbound and outbound. For inbound emails, it validates the signature for incoming emails. When sending outbound emails, it signs outgoing emails with your signature. It is supported in all 3 modes (stand-along, firewall & hybrid)

DKIM can only be used for outbound emails when using the Community Edition, unless you have less than 5 users. Once the number of users go beyond 5, filtering will be disabled and therefore, DKIM will not work for inbound messages.

Yes, you can install Xeams on CentOS