How to exclude a cache table from memcache?
You do it in the same way the  code you are showing does for cache_form.

$conf['cache_class_cache_abc'] = 'DrupalDatabaseCache';


Adding that line in the settings.php file, you are telling Drupal to save the cache for cache_abc in the database.

Keep in mind that, in Drupal cache terminology, cache_abc (which would be the name of the database table containing the cache data, when the cache is saved in the database) is called bin.