adding quantity to related products in magento backend

I'm new to magento and trying to add quantity column to the grid in related products tab (edit product -> related products). this is what I did: - overwrite related.php file:

Mage\Adminhtml\Block\Catalog\Product\Edit\Tab\Related.php

added to _prepareCollection() method this code:

$collection->joinField('qty','cataloginventory_stock_item',
                                    'qty',
                                    'product_id = entity_id',
                                    '{{table}}.stock_id = 1',
                                    'left'
                                    );

and added to _prepareColumns() methods this code:

$this->addColumn('qty', array(
            'header'    => Mage::helper('catalog')->__('QTY'),
            'width'     => 80,
            'index'     => 'qty'

now I can see the new column but the quantity is float number (for example 100.00) and I can't filter results based on my new QTY column. my questions: 1- is that all I need to add column or I have to do some thing else?? 2- how to display QTY in integer format (for example 100 not 100.00)?? 3- why I can't filter results based on the QTY?? any idea will be appreciated, thanx in advance..