To be used with ArcGIS, all users who will create data in or import data to the geodatabase must have a schema name that matches its user name. Even though the database itself allows multiple users to write to one schema, ArcGIS can't work with that. That is true no matter what your underlying database version is.
If you have users who only edit data (they don't create data; some other user creates it and they just have rights to edit the data), those users don't have to have matching schemas. Same thing for read-only users. You just have to be careful that you don't later grant those users the privileges necessary to create data...at that time, their data creation efforts will fail because the schema names don't match the user names.
Hope this helps.
Kim