Hi StevenZS,
yes, you are correct that you would need to purchase one Datacenter license for the cluster, which would cover unlimited Operating System Environments (OSEs) on the licensed servers.
Regarding your question, you can install both Standard and Datacenter editions of Windows Server on the licensed servers and still be covered by the Datacenter license. The Datacenter license allows for unlimited virtualization rights for Windows Server on the licensed servers, regardless of the edition of Windows Server being used.
However, it's important to note that the features and capabilities of the Standard and Datacenter editions of Windows Server differ, so you should ensure that the edition you choose for each virtual machine meets your specific needs and requirements.
Hope it helps.
Best regards,
Lei