As stated in:

http://post-office.corp.redhat.com/archives/sme-eap/2015-October/msg00077.html

My perception is that EAP 7 DC should be able to administer EAP 6 instances.

This is one of the most important feature my customer (which is participating in early access program) is looking for, so is crucial to understand if and when this will be supported.

I did a test, but EAP 6.4 is unable to connect to EAP 7 domain controller, and the following stack traces are logged:
6.4 (slave):

[Host Controller] 12:33:01,154 ERROR [org.jboss.as.controller.management-operation] (Host Controller Service Threads - 2) JBAS014612: Operation ("resolve-subsystems") failed - address: ([]) - failure description: "JBAS010877: Failed to load module 'org.wildfly.extension.batch.jberet'."
[Host Controller] 12:33:01,226 WARN [org.jboss.as.host.controller] (Controller Boot Thread) JBAS010900: Could not connect to remote domain controller at remote://127.0.0.1:9999 – java.io.IOException: java.util.concurrent.ExecutionException: Operation failed
[Host Controller] 12:33:02,263 ERROR [org.jboss.as.controller.management-operation] (Host Controller Service Threads - 2) JBAS014612: Operation ("resolve-subsystems") failed - address: ([]) - failure description: "JBAS010877: Failed to load module 'org.wildfly.extension.bean-validation'."
[Host Controller] 12:33:02,275 WARN [org.jboss.as.host.controller] (Controller Boot Thread) JBAS010900: Could not connect to remote domain controller at remote://127.0.0.1:9999 – java.io.IOException: java.util.concurrent.ExecutionException: Operation failed

....

7.0 beta (domain controller):

[Host Controller] 12:33:01,199 ERROR [org.jboss.as.controller.management-operation] (Host Controller Service Threads - 52) WFLYCTL0013: Operation ("read-master-domain-model") failed - address: ([]): java.lang.IllegalStateException: java.lang.InterruptedException
[Host Controller] at org.jboss.as.host.controller.mgmt.HostControllerRegistrationHandler$RegistrationContext.executeBlocking(HostControllerRegistrationHandler.java:617)
[Host Controller] at org.jboss.as.host.controller.mgmt.HostControllerRegistrationHandler$RegistrationContext.processSubsystems(HostControllerRegistrationHandler.java:456)
[Host Controller] at org.jboss.as.host.controller.mgmt.HostControllerRegistrationHandler$RegistrationContext.access$900(HostControllerRegistrationHandler.java:334)
[Host Controller] at org.jboss.as.host.controller.mgmt.HostControllerRegistrationHandler$HostRegistrationStepHandler.execute(HostControllerRegistrationHandler.java:327)

....