Thanks a lot this is what I was looking for. I start to make it work right now. The design is nearly done. But currently I have one additional question coming up:
I’am struggling right now integrating The Grid into my _index.php file so that archives, searches or categories got the same structure. Could you kindly give my a suggestion what I am doing wrong?
This is the code I am working with:
<?php
// =============================================================================
// VIEWS/GLOBAL/_INDEX.PHP
// -----------------------------------------------------------------------------
// Includes the index output.
// =============================================================================
$stack = x_get_stack();
if ( is_home() ) :
  $style     = x_get_option( 'x_blog_style' );
  $cols      = x_get_option( 'x_blog_masonry_columns' );
  $condition = is_home() && $style == 'masonry';
elseif ( is_archive() ) :
  $style     = x_get_option( 'x_archive_style' );
  $cols      = x_get_option( 'x_archive_masonry_columns' );
  $condition = is_archive() && $style == 'masonry';
elseif ( is_search() ) :
  $condition = false;
endif;
?>
<?php if ( $condition ) : ?>
  <?php x_get_view( 'global', '_script', 'isotope-index' ); ?>
  <div id="x-iso-container" class="x-iso-container x-iso-container-posts cols-<?php echo $cols; ?>">
    <?php if ( have_posts() ) : ?>
      <?php while ( have_posts() ) : the_post(); ?>
        <?php if ( $stack != 'ethos' ) : ?>
          <?php x_get_view( $stack, 'content', get_post_format() ); ?>
        <?php else : ?>
          <?php x_ethos_entry_cover( 'main-content' ); ?>
        <?php endif; ?>
      <?php endwhile; ?>
    <?php else : ?>
      <?php x_get_view( 'global', '_content-none' ); ?>
    <?php endif; ?>
  </div>
<?php else : ?>
  <?php if ( have_posts() ) : ?>
    <?php while ( have_posts() ) : the_post(); ?>
      <?php x_get_view( $stack, 'content', get_post_format() ); ?>
    <?php endwhile; ?>
  <?php else : ?>
    <?php x_get_view( 'global', '_content-none' ); ?>
  <?php endif; ?>
<?php endif; ?>
<?php pagenavi(); ?>
and in the documentation of The Grid it is mentioned that replacing everything between the while functions with this code  (Documentation)
The_Grid('My Grid Name', true); // where true is for template mode
(I replaced the Grid name with a empty grid with my skin settings). But I get an error that nothing could be found and this message is looped indefinitely - so something is wrong with my conditions, right?
I know this is kind of out of your scope of support but I would really appreciate it, if you could make a brief suggestion or tip where my mistake is.
Have a wonderful day!
Raphael