Hello Amit,
The agent can be on 6.5 version and they will work fine, to my best knowledge.
The only concern is that in case if you in future are getting some agent related fix then it can only be applied via upgrading from NAC in which case you need to upgrade the specific agent where you need fix to be applied.
However, as mentioned above it will be conditional, and can be live with until a fix related to agent is available in some releases.
Regards,
Saurabh