Yes, but I would change
$form['#validate'][] ='customfunctions_form_validate_custom';
to
$form['#validate'][] ='MYMODULE_user_registration_form_validate';
and
function MYMODULE_form_validate_custom($form, &$form_state) {
to
function MYMODULE_user_registration_form_validate($form, &$form_state) {
Nothing wrong with your code, but this just keep with the Drupal naming convention ie search_form_validate, node_form_validate, etc