You can do it up to 5 hours before the departure, usually there's no problem in finding a place.

> 2. Does anyone know how much a sleeper costs? Since I will have a railpass I just need to know how much extra a sleeper would be.

Approximately around 30$ per person.

> 3. If a sleeper accomodates 4 people but there are only 3 of us travelling, is the 4th spot filled by a stranger?

Yes, and sometimes there are even 6 in a car.

Ciao Flavio in Rome