Two lights are better, but if you use thee pendants, then position the two stools should be in the middle of the center and end lights.Although they will look very huge. Pictures would help.

I wish I could post links to pics of others' houses that are already on HOUZZ. Does anyone know how? I've been inspired by a project called "San Francisco Mid-Century Mix" on here.

400mm sounds good! Did you go for a smaller pendant?

I think it depends on the style and atmosphere you want to create in the room. If the room is a formal, traditional one like your picture, then one large showy pendant would look great. However if it is a modern or contemporary room then two ( or more) smaller pendants would be a nice feature. It would also pay to think about how much light you are expecting from the fitting, especially since it is a cathedral ceiling. Do you need the ceiling lights for good general lighting or just for atmosphere. Do you have any photos of your room so we can see a bit more?