We use Windows Server Datacenter within a cluster. Do we need to assign a Windows Server Datacenter license to every server within the cluster or can we also use other Windows Server editions, such as Windows Server Standard?

There is no rule specified by Microsoft stating that you have to license every server within a cluster with the same Windows Server edition. Different Windows Server editions can be active within a cluster as long as they are used according to the license use rights.

Hence, with a Windows Server Datacenter edition you may run any number of OSEs (Operating System Environments) on the licensed server. With a Windows Standard edition, the maximum is two virtual machines per server. If virtual machines are being moved within the cluster, the Windows Server Standard machines can only have up to two (2) virtual machines. Moreover, you may only use the software covered by the license. A Windows Server Datacenter licenses allows to run Windows Server Standard or Essentials; a Windows Server Standard license allows to (only) run Windows Server Essentials.

Source: Microsoft Product Terms, Oktober 2018