I created a mixed cluster with 6.6.1 (in Vagrant) and a local build of 7.0.
When I connect to the UI on the 7.0 node, if I go to the Analytics section, the code requests info about links on /_p/cbas/analytics/link, which returns an HTTP 401 error, which forces the UI to reload. This REST endpoint is used in both 6.6 and 7.0, so I don't know why it wouldn't work in a mixed cluster. I'm logged in as Administrator.
From the http_access.log: