Recommended release or Mature version to FortiNAC

You can use the latest version of 7.4.2 or 7.6.5 but remember that downgrade is not supported and reverting HW appliances to the previous version is not that easy compared to a setup running on a VM that can be quickly restored from a backup.
I would suggest to wait for 7.4.3, planned to be released on February and before the upgrade make sure to backup the setup as shown here: Technical Tip: FortiNAC-F system restore procedure
and check also the release notes of the firmware version for any known issues.