Latest posts feed with a specific post always first

Since you’re using get_posts() rather than WP_Query, it’s sufficient to retrieve the specific post separately with get_post(), and then add it to the beginning of the results array with array_unshift():

$postslist     = get_posts( $args );
$featured_post = get_post( 123 );

array_unshift( $postslist, $featured_post );

foreach ( $postslist as $post ) : setup_postdata($post);