How To Modify The Loop in archives.php To Have 11 Posts Per Page and CSS Styling

I edited my loop in archives.php in order to work with pagination and with the Category Post List Widget. I basically pieced it all together from random code and ideas and am now so far from the default post query that I’m having trouble getting back to square one. The reason I need to get back to square one is that I need two things:

1) My code is messing up my category posts when the post has two category tags. When that happens, they get placed in the wrong categories (or don’t get placed at all).

2) By doing this custom coding, I lost the query for ‘tags’ as well so the tag pages don’t work correctly.

I need to get back to square one so I can set up tags and category posts to be displayed correctly but within the correct <li>s that I style. I’m not sure if I even need the Category Post List widget anymore because, if I’m not mistaken, I can call the set_post_thumbnail_size();, the_title();, the_excerpt(), etc. without the widget and just style my own list items.

Anyhow, here’s the code I’ve got, I imagine I need to scrub a bunch of it to get back where I need to go. Thank you so much for the help, I really appreciate it.

<?php if (have_posts()) : ?>

<ul style="list-style-type:none;">

<?php
if ($paged == 0)
  $offset = 0;
else
  $offset = ($paged - 1) * 11;
global $post;
$category = get_the_category($post->ID);
$category = $category[0]->cat_ID;
$myposts = get_posts(array('numberposts' => 11, 'offset' => $offset, 'category__in' => array($category), 'post__not_in' => array($post->ID),'post_status'=>'publish'));
foreach($myposts as $post) :
setup_postdata($post);
?>


<li id="category_li">
    <div id="image_con">
        <div id="image_recent">
        <a href="<?php the_permalink(); ?>"><?php set_post_thumbnail_size( 200, 200 );
the_post_thumbnail(); ?></a>
        </div>
    </div>

    <div id="cr_content">
    <a href="<?php the_permalink(); ?>">
    <div class="gloss_glam_font"> <?php the_title(); ?></div>
    <hr>
    <div class="excerpt_cat"><?php the_excerpt(); ?></div>
    </a>
    </div>

    <div id="clearfix">
    </div>
</li>

<?php endforeach; ?>
<?php wp_reset_query(); ?>
</ul>


<?php while (have_posts()) : the_post(); ?>

<?php endwhile; ?>

<?php if (function_exists("pagination")) {
    pagination($additional_loop->max_num_pages);
} ?>
    <?php else :

        if ( is_category() ) { // If this is a category archive
        printf("<div class='post-content'><p><em><strong>Not Found:</strong> Sorry, but there aren't any posts in the &middot;<strong>%s</strong>&middot; category yet.</em></p></div>", single_cat_title('',false));
    } else if ( is_date() ) { // If this is a date archive
        echo("<div class='post-content'><p><em><strong>Not Found:</strong> Sorry, but there aren't any posts with this date.</em></p></div>");
    } else if ( is_author() ) { // If this is a category archive
        $userdata = get_userdatabylogin(get_query_var('author_name'));
        printf("<div class='post-content'><p><em><strong>Not Found:</strong> Sorry, but there aren't any posts by &middot;<strong>%s</strong>&middot; yet.</em></p></div>", $userdata->display_name);
    } else {
        echo("<div class='post-content'><p><em>No posts found.</em></p></div>");
    }

endif; ?>

EDIT

Alright, I edited it and I can’t figure out where I’m off. I’m using my tag.php template as an example here (not the category.php). Here’s what I’ve done so far (based off the code provided by Chip in the answers):

function wpse63424_filter_pre_get_posts( $query ) {
    if ( ! is_main_query() ) {
        return $query;
    } else {
        if ( is_category() || is_tag() ) {
            $query->set( 'posts_per_page',11 );
        }
        return $query;
    }
}
add_filter( 'pre_get_posts', 'wpse63424_filter_pre_get_posts' );

$tag = get_query_var('tag');
$myposts = get_posts('tag__in' => array($tag));
foreach($myposts as $post) :
setup_postdata($post);

My thinking was that I’ve set a filter of 11 posts per page, I query for a specific tag, set the posts to the tag in that array and then set up the posts accordingly. When I just run get_posts(); it returns 11 posts per page unfiltered so I know that Chip’s code works fine. I guess I’m not understanding how the functions are called correctly yet…