Schema Browser user/schema dropdown list does not show all users/schemas.
There is a filter created under "Oracle Users List" option of Session menu which also filters the user/schemas listed in the dropdown list.
1. In versions 9.0.0 and above select Session | Oracle Users List menu option. In versions 8.6.1 and below select View | Oracle Users List menu option.
2. Select the users to be displayed in the dropdown list of Schema Browser and select OK.
3. Select "Refresh ALL Lists" icon of Schema Browser in order to refresh the dropdown list.
Applied to all Toad for Oracle versions.
This is not a setting in the Toad.ini file.