What happens:
While trying to run domain with EAP 7 with EAP 6 as slaves the following happens:

Intro:

On running following cli commands after domain controller started, the jgroups get modified by themselves in a wrong way:
------
cli commands run:
/profile=MP_NONSINGLETONS/subsystem=bean-validation:remove
/profile=MP_TRANS_RMP_RAY/subsystem=bean-validation:remove
-------
Before running cli command:
<protocol type="pbcast.NAKACK"/>
<protocol type="UNICAST2"/>
After running cli command:
<protocol type="pbcast.NAKACK2"/>
<protocol type="UNICAST3"/>

------
Full jgroups section before running cli command for MP_TRANS_RMP_RAY, MP_NONSINGLETONS profiles:

Before running cli command:
<subsystem xmlns="urn:jboss:domain:jgroups:1.1" default-stack="udp">
<stack name="udp">
<transport type="UDP" socket-binding="jgroups-udp"/>
<protocol type="PING"/>
<protocol type="MERGE3"/>
<protocol type="FD_SOCK" socket-binding="jgroups-udp-fd"/>
<protocol type="FD"/>
<protocol type="VERIFY_SUSPECT"/>
<protocol type="pbcast.NAKACK"/>
<protocol type="UNICAST2"/>
<protocol type="pbcast.STABLE"/>
<protocol type="pbcast.GMS"/>
<protocol type="UFC"/>
<protocol type="MFC"/>
<protocol type="FRAG2"/>
<protocol type="RSVP"/>
</stack>
</subsystem>

After running cli command:
~~~
<subsystem xmlns="urn:jboss:domain:jgroups:6.0">
<channels default="auto">
<channel name="ee-web" stack="udp"/>
<channel name="ee-hibernate" stack="udp"/>
<channel name="auto" stack="udp"/>
</channels>
<stacks default="udp">
<stack name="udp">
<transport type="UDP" socket-binding="jgroups-udp">
<property name="port_range">
50
</property>
</transport>
<protocol type="PING"/>
<protocol type="MERGE3"/>
<protocol type="FD_SOCK" socket-binding="jgroups-udp-fd"/>
<protocol type="FD"/>
<protocol type="VERIFY_SUSPECT"/>
<protocol type="pbcast.NAKACK2"/>
<protocol type="UNICAST3"/>
<protocol type="pbcast.STABLE"/>
<protocol type="pbcast.GMS"/>
<protocol type="UFC"/>
<protocol type="MFC"/>
<protocol type="FRAG2"/>
<protocol type="RSVP"/>
</stack>
</stacks>
</subsystem>
~~~
The architecture EAP 7 master - EAP 6 slave is supported according to https://access.redhat.com/articles/2568341