Q: What series virtual machines can be mixed within an Availability Set in Azure?
A: Under normal circumstances, the virtual machines in an Availability Set should be of similar configuration. They're providing a like service and therefore will probably be the same virtual machine series and size because you want a similar experience for users of the service. However, there might be times when you want to mix various sizes and series of virtual machine.
Azure deploys servers in scale units, also known as stamps and clusters. All hosts in a scale unit are the same hardware (e.g., processor, type of local storage). A cloud service is bound to a specific scale unit and an Availability Set resides in a single cloud service. Therefore, all virtual machines in an Availability Set are bound to a specific scale unit; the exact virtual machines available depend on the type of scale unit the cloud service is bound to. See "Virtual Machine Size/Series Limitations when Adding/Resizing to a Cloud Service" for more information.