For Medium and SMall, the intervals (1800 and 7200) correspond respectively with HI2 and HI3.
For Large I'm not sure what you mean by the '*' value, that is normally used for the Instance parameter.
Correct CounterName is the same as Metric.
You pass the required metrics via the Stat parameter.
Yes, I would suggest to open a SR for the issue with Get-Stat on vSphere 6.0.
If GSS claims you need a Developer Support Contract to open calls for PowerCLI, point them to PowerCLI Support Breakdown.
You don't need a Developer contract when the issue is obviously a bug in a cmdlet, and not in your script.