I was using social engine and I don't wanted to change my core helpers after seeing the code I find the following solution which make some selected textarea as tinymce editor and the remaining textarea remains same,
$form = new Engine_Form();
$form->addElement('Tinymce', 'birthdate', array('editorOptions' => array('theme_advanced_buttons1' => array('undo', 'redo', 'cleanup', 'removeformat', 'pasteword', '|', 'code',
'media', 'image', 'link', 'fullscreen', 'preview'), 'editor_selector' => 'mceEditor')))->setAttrib('class', 'mceEditor');
$form->birthdate->setAttrib('class', 'mceEditor');
Here editorOptions array key sets tinymce configs.