If you are using admin generator as you indicated, you can edit your forms via a generator.yml file. With this file you can do any number of things include set which widgets you want to appear, order of entry, actions, etc.
The generator.yml file is located in /apps/app_name/modules/module_name/config
You can read more about it in the symfony docs.
-- Edit --
If you are not using the generator.yml file, you can edit the form class directly, read this article relating to symfony forms for more info.
Example widget manipulation:
//-----
//Remove Unwanted
//-----
unset(
$this['created_at'],
$this['updated_at'],
$this['ingredient_list'] //Will be embedded due to extra meta data
);
//-----
// Add a select menu using model to populate values
$this->widgetSchema['state_list'] = new sfWidgetFormPropelChoice(array('model' => 'State', 'multiple' => true, 'order_by' => array('name', 'asc')));
// Add matching validator
$this->validatorSchema['state_list'] = new sfValidatorPropelChoice(array('model' => 'State', 'column' => 'id', 'multiple' => true));
// I can also force widget presentation order
$this->getWidgetSchema()->moveField('country_list', sfWidgetFormSchema::AFTER, 'state_list');
// You can also add a callback function when the form is submitted
$this->validatorSchema->setPostValidator(
new sfValidatorCallback(array('callback' => array($this, 'dupeCheck')))
);