How to add css class to these form fields?
You should write a statement that first check the form element type then add class attrubutes to it,
something like 
//in form_alter

foreach($form as $form_elem){
if($form-elem['#type']=='textfield'){
    $form[$form_elem]['#attributes']['class'][] = 'myclass';
   }
}


(this is not the exact solution,I just want give you idea)

another way is using hook_preprocess_field 

and the final and easy way is using js to add class, because drupal set default class for his textfields,you just need one line jquery code to add you 

   $('.form-text').addClass('myclass');


if you don't want this js not load in all page, I suggest you add your js with drupal_add_js conditionally in hook_form_alter, it means that  

    //in hook_form_alter
     $des_forms=array('node_add_page','node_add_prodcut');
    if(in_array($from_id,$des_forms)){
    drupal_add_js('your js'); //you also add it inline,but I suggest you put it in js and add it as file
}


again I tell you because I have no time in this moment I just suggest you some solution idea, I hope could help you