You could put a conditional IF around the display of the Units based on master_categories_id for that Seconds Category ...
You can customize the template using your overrides so that if the Seconds categories_id is 10, then use:
Code:
<?php
$show_seconds = $db->Execute("SELECT master_categories_id from " . TABLE_PRODUCTS . " WHERE products_id ='" . $product_info->fields['products_id'] . "'");
if ($show_seconds->fields['master_categories_id'] == 10) {
// show quantity
$show_quantity_seconds = true;
} else {
// do not show quantity
$show_quantity_seconds = false;
}
?>
<?php echo ($show_quantity_seconds && ($flag_show_product_info_quantity == 1) ? '<li>' . $products_quantity . TEXT_PRODUCT_QUANTITY . '</li>' : '') . "\n"; ?>