Comment by Fysi

You absolutely can mix Google Workspace licences but you either need to go via the Google sales team or via an Google services partner.

And to be honest, I understand the reasoning, Google Workspace is supposed to be simpler to operate in comparison to Microsoft 365, and simple means a single licencing choice.

If you're at the scale where you need to worry about mixing and matching licences, you're at the scale where you should be looking at either an enterprise contract or going through a partner, otherwise you're going to be paying list price.

It sucks for people who can handle the complexity, but also as someone who has spent far too time in Microsoft licencing conversations and contract talks, Microsoft licencing is painfully complex (some licencing is across the whole tenant, some spend is consumption based billing against Azure, some licencing is requires a minimum amount against a %age of your total M365 user base, and then don't get me started on licencing machine users or multiplexing).