You don't need Role Scoper to accomplish that. Just use a basic role editing plugin such as Capability Manager or Members to create a custom role. Then give your custom role the capabilities install_plugins and activate_plugins.

To prevent Role Scoper from treating that role as a content administrator, give your Administrator role a new custom capability administer_all_content and add this to wp-config.php:

define( 'SCOPER_CONTENT_ADMIN_CAP', 'administer_all_content' );