Answered VIA Support.
It is ok to install it to the Principle server.
The reason it is asking for an authentication server is because the all authentication happens via the Print Server Service which is not on a Principle Server.
Cory, In the event that a print server goes down, be aware that there is a registry key that can be edited/added to specify a backup server. 32-bit and 64-bit key locations (respectively) HKEY_LOCAL_MACHINE\SOFTWARE\Pharos\Edi HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Pharos\Edi Name: BackupServer Type: REG_SZ Info: The host name of the Pharos Server that the EDI will use if the main server is unavailable. This value must be manually created. Regards, Steven