The main motivation of the Mix-and-Match feature is to enable Maestro customers with older appliance to expand and grow their Security Groups with newly introduced models.
The Mix-and-Match feature was not designed to enable any combination of models, but aimed to enable mix of appliances from the same family or size (i.e. similar computing power) along the supported generations. It means that we enable small models with small models old & new, medium models with medium, etc.
We are working to improve the documentation/Sk around it to make things clear and it should be available in the coming week