Windows Server 2012, while it's technically possible to team network cards of different speeds into a single NIC team, it's not a good idea for several reasons.
It's not supported by Microsoft.
The native NIC teaming solution algorithms don't take the speed of the NIC into consideration when balancing traffic and instead just balance traffic evenly between adapters. This means if a single team had a 1 Gbps adapter and a 10 Gbps adapter, each adapter would get half of the traffic, which isn't optimal.
On a similar point to number 2, should adapters of a higher bandwidth fail, then performance would drop as the lower bandwidth adapters are used, affecting user experience.