If we set up Federated ID, does it have to be done for all users in the Admin console? Or can some users be left as Adobe ID others Federated?
You can add some users with a federated and some with Adobe IDs. It is not compulsory to add all of them with either federated or Adobe ID in the admin console.
You can set up in a way that has a mix of users with different Identity types.
However, it'll also depend on the plan/product that you have. For example, if your organization has Enterprise storage (i.e you see a Storage tab in your Admin Console), you won't be able to assign Creative Cloud products to users with Adobe IDs. See Identity overview for more details.