I understand that a Windows Server 2012 R2 Essentials instance must be the domain controller and hold all FSMO roles. I am told that one can domain-join other Windows Servers (Standard or Datacenter) to a domain and use them as domain controllers, SQL servers, or whatever so long as the Essentials server maintains FSMO roles. My question is, rather then going with Standard or Datacenter edition (and thus also needing CALs), can I domain-join other Essentials servers for SQL Server and other roles? The way I understand it, so long as an instance of Essentials holds the FSMO roles, any other Windows Server edition can be domain-joined and used for any purpose. Is his correct?
If any Essentials Server does not hold the FSMO of its domain, then it will regularly and automatically shutdown.
The Windows Server 2012 R2 Essentials once deployed, must be the domain controller at the root of the forest and domain, and must hold all the FSMO roles.
This will not work, the essentials server ( as opposed to essentials experience added to a standard server) must be the root of forest and domain, in this instance if you join another it would have to be a separate domain and forest. The admin overhead of having two separate like this is very high. If you want another DC then use standard. If you want a member server added also use standard.
You only need datacenter if you have huge hardware and want to virtualise more than two VM's