Question regarding index posts and pagination

Hello there! I’ve a site running the last version of WordPress 4.4 and want to make a modification regarding the number of post displayed on index.php than pagination.

The problem is that the index.php page must show 7 posts and the pages of pagination (/page/2 – /page/3 – /page/4) must show 8 posts. I’m trying to do this modification but can’t get it to work.

Also the first post on the index.php page must have a different style but not on the paginated version. My actual code is:

<?php if (have_posts()) : ?>
<?php $post = $posts[0]; $c=0; ?>
<?php query_posts($query_string . '&cat=1,2'); ?>
<?php /* Start the Loop */ ?>
<?php while (have_posts()) : the_post(); ?>
<?php $c++;
if( $c == 1) :?>
STYLE FIRST POST

<?php else :?>
<?php get_template_part( 'content', get_post_format() ); ?>
<?php endif;?>
<?php endwhile; ?>

Thanks very much in advance! =D