Could we have for instance Zeppelin 0.7 from HDP 2.6 but rest of the service remains at the previous version (HDP 2.5)
Ambari currently only supports upgrading the entire HDP stack together. Individual component upgrades are not supported.
However, we are planning "patch upgrades" (https://issues.apache.org/jira/browse/AMBARI-12556) which would allow you to upgrade only a single component of the stack. This is targeted for the Ambari 3.0 release.