We have a requirement to add few datanodes to existing hdp cluster. The existing cluster has datanodes running on Redhat 6.6 version. The new datanodes that are to be added are running on Redhat 6.8 version. Can I add them to the existing cluster as datanodes.
Yes, you could have different minor version of OS on same cluster. For example as you have mentioned you could have any RHEL 6.x versions of host in a cluster. But mix of versions such as RHEL 6.x and RHEL 7.x is not recommended (though it is possible with lot of own risk)
Following link would help in answering the mix of OS.
You should be able to add the datanodes [ running on Redhat 6.8] to an existing
cluster [ with datanodes running on Redhat 6.6]. The difference in minor
version of the OS should be fine as the repo used, will be same. i.e. Linux 6.x
to add node to an existing cluster via ambari :