Is it possible to have a Splunk environment with a mix of 5.0.x and 6.0.x versions?
Specifically have all ES components running the currently compatible 5.0.x version like the master node, indexers, search heads and have other components running 6.0.x like master license node, deployment server, universal forwarder, and other apps.
Just some housekeeping in answers... ES 3 is out now and works on Splunk 6.
To the original question, no, it's not advised to mix components for this purpose.