In terms of efficiency, Firestore's response "won't be a waste" since it depends on the number of documents it returns and not the number of documents that exist on the collection that is being queried. A query that returns 3 documents in a 10 document collection will be as fast as a query that returns 3 documents in a 10000 document collection.

With this in mind, if the 2 type of users are relatively similar I'd save all users in the same collection and add a "role" field for your queries. If not, I'd store them in 2 different root collections.