I was looking for the same info and after digging, I found out that caps added must match capability_type value ('custom' above).
So if you set 'capability_type' => 'custom', the code should be:
$role->add_cap( 'delete_customs', true );
$role->add_cap( 'edit_customs', true );
$role->add_cap( 'edit_others_customs', true );
$role->add_cap( 'publish_customs', true );
$role->add_cap( 'read_private_customs', true );
// ...
(note cap type is singular and caps added are plural)
But if you set `'capability_type' => 'news_post', then you can use:
$role->add_cap( 'delete_news_posts', true );
$role->add_cap( 'edit_news_posts', true );
$role->add_cap( 'edit_others_news_posts', true );
$role->add_cap( 'publish_news_posts', true );
$role->add_cap( 'read_private_news_posts', true );