Q. I want to create a VM in Azure with multiple vmNICs. Can I mix them with VMs with single vmNICs?
A. A recent update to Azure gave customers the ability to have VMs with multiple vmNICs (the exact number depends on the size of the VM) with each vmNIC residing in a separate virtual subnet. It is fully supported to have VMs with multiple vmNICs and VMs with a single vmNIC habitating the same virtual networks and virtual subnets and this would be expected.
The restricting is on the deployment. You cannot mix multi-vmNIC and single vmNIC VMs in the same deployment. You'll have to create separate cloud services for single vmNIC VMs and for multi-vmNIC VMs.
(Editor's note: John wrote more about Azure vmNICs here.)