Custom pagination (Previous / Next)

Well… pretty much just do exactly what you said. Add a class conditionally based on the number of links O.o

$link_container_class="";

if (!empty($links)) { 
  if( 1 === count( $links ) ) {
    $link_container_class="single";
  }
  ?>
  <nav class="post-nav clear clearfix <?php echo $link_container_class; ?>">
    <ul><?php
      echo implode($links); ?>
    </ul>
 </nav><?php
} ?>