Memcache and form cache - is specfiying database for form cache still necessary?
Drupal 8 does not have a {cache_form} table; in-progress $form_state got moved to the key-value store.  The KV store for forms defaults to the database (there are core classes to support database and memory for KV).  And the KV store and cache backends are different concepts in Drupal 8 (the former is for persistent data with expiration, the later is for ephemeral data with expiration), so you should be fine.