MSC Poesia Specialty Dining

Hi

I am booked on the MSC Poesia for the end of August for 7 nights around the Greek Islands, just been looking on the website for excursions and I noticed you can book the Specialty Dining Sushi restaurant, do you need to book this before the cruise or can you easily get availability during the cruise?

I've only cruised with RCI before and they usually have a few specialty restaurants like a Steak, Italian & Burger Restaurant which we reserved while on board, does the Poesia just have Sushi?

Also, I believe there is a Gelato shop, is this part of the standard restaurants, or paid for extra?

Thanks for any advice.