Final answer:
To add custom virtual machines to the Azure virtual machine scale set VMSS1, you should change the orchestration mode from Uniform to Flexible, as Flexible mode allows for each VM instance to be uniquely configured.
Step-by-step explanation:
You mentioned that you plan to create a virtual machine scale set (VMSS) in Azure and would like to add custom virtual machines to it. The orchestration mode setting determines how virtual machines are managed within the scale set. Since you want to add custom VM instances, you should modify the orchestration mode from Uniform to Flexible. The Uniform mode does not allow for custom VM instances as it uses a single VM image to scale out all instances. In contrast, the Flexible orchestration mode allows each VM instance to be uniquely configured, which would permit the addition of custom VMs to your scale set VMSS1.