It is possible to use Google Identity Platform and Firebase Auth concurrently in the same project without upgrading my email/password users in Firebase Auth to Google Identity Platform and having to pay for those users?
Attempting to use Google Identity Platform seems to force me to move all my users in Firebase Auth over to it.
As far as I know it is indeed not possible to use Google Cloud's Identity Platform and Firebase Authentication on a single project. You'll have to pick one, and use that for the entire project.
The only alternative I can think of is setting up separate projects for each, and accessing the specific project for the type of user. But in that scenario you'll have to deal with data migration and probably more nasty situations.
Update: since June 2022 the upgrade to Identity Platform is available directly within Firebase. While this optional upgrade does switch you to a different pricing plan which is no longer unlimited, it comes with a generous free tier of 50,000 MAU (monthly active users).
For more on this see the announcement blog post MFA, Blocking functions, and more come to Firebase Authentication and the documentation section on Firebase Authentication with Identity Platform.