There should be no major issues with installing this from 6.1/2 into 6.3 or 6.4.
Have you installed and are having problems?
No, but I usually wait for the version I'm on to be supported before trying to install Apps or Add-ons.
I have seen one issue which I could use some help or guidance on how to fix, occasionally I get where all the dashboards show "No results found", the last time I think I resolved it by rebooting all the indexers and search heads, but that's a pain. I tried rebuilding the data model for the app but that didn't resolve it, any other suggestions?
An update to the Cisco Security Suite app (ver 3.1.2) was recently published to SplunkBase: https://splunkbase.splunk.com/app/525/
This version supports Splunk 6.3.x and 6.4.x and the latest versions of all required Add-ons (see change log in release notes ).