Frederik's apartment is wonderful. It's basically two apartments stuck together, so it's great for larger groups. There's both a kitchen and a kitchenette, there are interior doors for privacy, two living room spaces, and plenty of bathrooms for everyone.
The location is epic -- just a stone's throw from the most beautiful sights in the city. Just be aware that it is right in the middle of the action, and there can be a good amount of noise from the street. Our infant son slept through it with no problems, but we had one traveling companion who is a very light sleeper and struggled a little.
Overall this is an amazing apartment and Frederik is a gracious and welcoming host. I would happily recommend this apartment to anyone traveling to Florence as part of a large group.