grazie a 2 funzioni trovate Googlando:

// elimino il prefisso dei post custom
function na_remove_slug( $post_link, $post, $leavename ) {

    if ( 'servizio' != $post->post_type || 'publish' != $post->post_status ) {
        return $post_link;
    }

    $post_link = str_replace( '/' . $post->post_type . '/', '/', $post_link );

    return $post_link;
}
add_filter( 'post_type_link', 'na_remove_slug', 10, 3 );


// seconda parte del codice
function na_parse_request( $query ) {

    if ( ! $query->is_main_query() || 2 != count( $query->query ) || ! isset( $query->query['page'] ) ) {
        return;
    }

    if ( ! empty( $query->query['name'] ) ) {
        $query->set( 'post_type', array( 'post', 'servizio', 'page' ) );
    }
}
add_action( 'pre_get_posts', 'na_parse_request' );