Implementing custom tag/tax pages

You can use the tag_template or taxonomy_template filters and check the queried object from within that, then use locate_template to get the template path.

function wpd_tag_template_filter( $templates="" ) {
    $this_tag = get_queried_object();
    // check $this_tag->term_id, $this_tag->slug, etc.
    $templates = locate_template( 'my_custom_template.php', false );
    return $templates;
}
add_filter( 'tag_template', 'wpd_tag_template_filter' );