Create multiple subclasses of Form Validation + callback inside should work

This is a two-part problem because I believe the issues are dependent on each other.

What I want to achieve:

I am creating a site with multiple specific forms and I want each of them to have their (1) own validation class instead of a bloated MY_Form_validation.php.

(2) Callbacks will also happen inside that validation class and not within the form's controller.

OK CASE:

I create MY_Form_validation.php with a callback method called "check_value":

PHP Code:

<?php
class MY_Form_validation extends CI_Form_validation
{
    protected $CI;

    public function __construct($rules = array())
    {
        parent::__construct($rules);
        $this->CI =& get_instance();
    }

    public function validate()
    {
        $this->CI->form_validation->set_rules('value', 'value',
            'trim|required|integer|check_value['.$this->CI->input->post('unit').']');
        $this->CI->form_validation->set_rules('unit', 'unit', 'trim|required');
    }

    public function check_value($value, $unit)
    {
        if($value >= 100)
        {
            return TRUE;
        }
        else
        {
            $this->CI->form_validation->set_message('check_value',
                'The %s must be greater than or equal 100.');
            return FALSE;
        }
    } 


 
check_value is called inside validate() and it is called without the "callback_" prefix. It works that way.

This validation class is used in the form controller as:

PHP Code:

public function __construct()
{
    parent::__construct();
    $this->load->helper(array('form', 'url'));
    $this->load->library('form_validation'); 
} 


and the method called:

PHP Code:

$this->form_validation->validate(); 


NOK CASE:

Same settings with OK CASE, just with a classname like "Car_registration_form_validation" and the manner of usage:

PHP Code:

public function __construct()
{
    parent::__construct();
    $this->load->helper(array('form', 'url'));
    $this->load->library('form_validation');
    $this->load->library('car_registration_form_validation');
} 


PHP Code:

$this->car_registration_form_validation->validate(); 


With the settings above, I get the following error:

Code:

Unable to access an error message corresponding to your field name my_field.(check_value)

My questions:

1. Is it possible to create more than one (aside from MY_Form_validation) form validation subclasses?
   
2. Is my way of calling the callback correct in the NOK CASE? Does it need a special way since its class name is "MY_Form_validation" (the only discrepancy I see)?