How to display elements of different post types?

I am not sure I understand your conditions but I think you want something like this:

$arg = array (
  'post_type' => array('post','placas'),
  'orderby' => 'rand',

  'tax_query' => array(
     'relation' => 'OR',
     array(
        'taxonomy'  => 'firmas',
        'field'     => 'slug',
        'terms'     => array( 'LG' ),
        'operator'  => 'NOT IN',
     ),
     array(
        'taxonomy'  => 'category',
        'field'     => 'slug',
        'terms'     => array( 'news' ),
        'operator'  => 'NOT IN',
     ),
   )
);

$query_slider= new WP_Query($arg);

You are including both relevant posts types in the query and you are altering your tax_query to return from either of two tax/slug combinations.

That brings me back to not being sure I understand your conditions. Depending on what you need, this may not work. You may need logic that WP_Query is not capable of, in which case you’d need one or more filters.