try this ( untested and assuming of course that $tag is the right object tag and $tag[‘count’] is the number of post with that tag ):
if ( $tag['count'] > 1 ) {
//first part of your code
}
else if($tag['count'] == 1){
$the_query = new WP_Query( 'tag='.$tag['name'] );
if ( $the_query->have_posts() ) {
while ( $the_query->have_posts() ) {
$the_query->the_post();
echo '<a href="';
echo get_the_permalink();
echo '">';
echo $tag['slug'];
echo '</a></br>';
}
}
wp_reset_query();
}
else{
//// last part of your code
}