How to find all schemas where a table has records

Using import with show=y won't actually do the import, but will show which tables the import will run against. There's no practical way, however, to show what rows import will write; it's assumed that you, running the import, would know that.

You may wish to look at some of Oracle's extensive documentation about these utilities to determine what options are available and what they do.

You have a few options, some of which will make your database unsupportable by Oracle (if you care!) as you need to fiddle with SYS owned objects.

The first option is to revoke select privs from everyone:

revoke select on all_users from public;

Then grant it back to certain users, plus all other internal Oracle users.

The 2nd option is to look at the source for the view, drop it, then recreate an altered version that supports your needs.

Another option that would trick casual users would be to create a table called ALL_USERS in each schema. Unless the logged in person explicitly qualified ALL_USERS (ie: SYS.ALL_USERS) they would get the resultset from the table in their own schema. This trick could also be done with a synonym pointing to a view that does:

select * from all_users where username = (SELECT sys_context('USERENV', 'SESSION_USER') from DUAL);

Not much else you can do with XE I'm afraid.