Additional form validation only after passing default validation In Drupal 6.25.

I am building a custom user registration form that needs to run some actions on an external service after properly passing default form validation but before creating a new user record in Drupal. So far I am doing the following:

function mbrusers_form_alter(&amp;$form, &amp;$form_state, $form_id){
   if($form_id == "user_register"){
       if($default_form_validation_passed){
           $form['#validate'][] = 'mbrusers_infusionConnect_validate';
       }
   }
}

function mbrusers_infusionConnect_validate($form, &amp;$form_state){
    // Run actions on external service
    if($error){
        form_set_error("", "SET ERROR");
    }
}


I feel like I'm overlooking something, but I haven't been able to figure out what to do. Maybe using validate is the wrong idea? Any help that sets me in the right direction is appreciated.
I'm pretty sure you have to re-arrange the order of the validators, and inject yours basically in the middle instead of just adding it at the end.

From user.module, $form['#validate'] = user_login_default_validators(); or:

/**
 * Set up a series for validators which check for blocked/denied users,
 * then authenticate against local database, then return an error if
 * authentication fails. Distributed authentication modules are welcome
 * to use hook_form_alter() to change this series in order to
 * authenticate against their user database instead of the local users
 * table.
 *
 * We use three validators instead of one since external authentication
 * modules usually only need to alter the second validator.
 *
 * @see user_login_name_validate()
 * @see user_login_authenticate_validate()
 * @see user_login_final_validate()
 * @return array
 *   A simple list of validate functions.
 */
function user_login_default_validators() {
  return array('user_login_name_validate', 'user_login_authenticate_validate', 'user_login_final_validate');
}


and

/**
 * A validate handler on the login form. Check supplied username/password
 * against local users table. If successful, sets the global $user object.
 */
function user_login_authenticate_validate($form, &amp;$form_state) {
  user_authenticate($form_state['values']);
}


so I'd do something like:

$form['#validate'] = array('user_login_name_validate', 'mbrusers_infusionConnect_validate', 'user_login_authenticate_validate', 'user_login_final_validate');


instead of if &lt;Default form validation passed&gt; so that your validation function is called before the normal/usual second validator.