Supportability of adding Azure VMs to an existing availability set
You may occasionally encounter limitations when you add new virtual machines (VMs) to an existing availability set. The following chart details which VM series you can mix in the same availability set.
Here is the supportability matrix to mix different types of VMs:
|Series & Availability Set||Second VM||A||Av2||D||Dv2||Dv3|
All other series could not be in the same availability set because they require a specific hardware.
A8/A9 VM size can't be mixed due to requirement on dedicated RDMA backend network.