Don't know how to word this question. Sorry

[eluser]pickupman[/eluser]
Congrats on finding CI, and welcome to the forums. You can create multiple if statements for your form validation rules.

Code:

$this->form_validation->set_rules('userID', 'user id', 'required|trim');
if(isset($this->input->post('userID'))
   $this->form_validation->set_rules('security_question1', 'security question', 'required|trim');

if($this->form_validation->run()){
  //Process reset password
}