I have Google Apps free plan (50 Google accounts) and I would like to apply a different plan for one account only: from free to 100 GB/month (lots of mails, I need space).
I fear that changing plan applies the change to all accounts, increasing the monthly cost x50 times.
Is my fear founded?
Yes. Unfortunately, I can't find the official documentation anymore, but a while ago I was also checking for mixing different user types in one G Suite domain, but unfortunately that was not possible.
Note also, that the free account is no longer available for new registrations, and only people who have an existing free account can continue to use it as such.
No, your fear is not founded.
G Suite allows you to:
Let users buy their own extra storage—Users can purchase their own extra storage if you turn on the Individual Storage service in your Admin console.
(See: G Suite Administrator Help article)
The billing for Individual storage is separate from billing for G Suite (formerly, Google Apps for Work). The Individual storage is managed via Google Play terms of service, while G Suite billing is managed by the G Suite agreement.
At the present time you can safely upgrade individual storage without affecting your legacy edition of G Suite.
