Change post count on tag/terms pages to 10

You’re almost there my friend. Just check with $query->is_tag() rather than only is_tag(). So your updated code will be-

function main_query_mods( $query ) {
    // check http://codex.wordpress.org/Conditional_Tags to play with other queries
    if(!$query->is_main_query()) {
        return;
    }
    if($query->is_tag()) {
        $query->set('posts_per_page',10);
    }
}
add_action( 'pre_get_posts', 'main_query_mods' );