Can other Windows Servers be in the same AD domain with a Windows Essentials server?

I understand that Windows Server Essentials 2012 R2 must be deployed as a the only domain controller in an environment. It cannot simply be used as domain-joined Windows Server running SQL or something. However, I cannot find information to confirm that the inverse is true. It's my understanding that any Windows Server Standard/Datacenter instance can be domain-joined to a domain hosted by Windows Server Essentials. Basically, so long as the Essentials server is the only domain controller, there shouldn't be a licensing problem (CALs aside) with additional Windows Servers, right?

I understand that Windows Server Essentials 2012 R2 must be deployed as a the only domain controller in an environment.

You can have a second DC with Essentials (as you could with SBS--there were entire TechNet articles about deploying your second server in an SBS environment). Essentials has to hold all the FSMO roles in your domain, but you can indeed have more than one DC with SBS or Essentials.

(This is a common and pervasive myth.)

So yes, you can have member servers, too.

Yes.

See How to buy Windows Server 2012 R2 Essentials

It is an ideal first server, and it can also be used as the primary server in a multi-server environment for small businesses to protect, centralize, organize, and access their applications and information from almost anywhere by using virtually any device.