I’ll introduce the response I received from Google official support regarding default settings for group permissions in G Suite (formerly Google Apps).

Default Permission Settings When Creating New Groups

Q) Is it possible to change the default permission settings when creating new groups?

A) As an example, when creating a new group from the admin console, you can change the default permissions by modifying the access level.

グループの詳細を表示または編集する - G Suite 管理者 ヘルプ

Super Admins and Admins Can View Group Forums

Q) Can super admins and admins be configured not to view group forums?

A) Even if you set topic viewing to group owners only and add G Suite administrators as members to the group, as stated in the help article below, they have full control, so it’s not possible to prevent forum viewing.

Google グループのセキュリティ - G Suite 管理者 ヘルプ

Settings to Not Keep Emails in Group Forums

Q) About settings to not keep emails in group forums

A) In the group screen, by unchecking “Archive messages to the group” under Manage > Information > Content Management > Archive Options, you can configure not to archive forums.

That’s all from the Gemba.