We have been in this apparment with 3 adults and this was totally fine (for 6 people it might be a little "cosy", but manageble). It's located on a street with traffic, but with closed windows you do not hear anything.
Metro station is 3 walking minutes away.
Nicolo is a great host and answers all questions on short notice and he had a lot of useful hints regarding our stay in Rome... ;) It was an absolutely great trip!
Coming back again - for sure…