From my experiences in Italy, it's not a problem to share if you explain. (Mi hanno operato allo stomaco e non posso mangiare molto... andrebbe bene se io con mio marito condividessimo il piatto principale, se ognuno de noi ordinasse un antipasto?)
However, you might have some trouble with protein in the morning. It depends where you are staying.
If you are in a rented apartment where you must shop for your own food, of course, no problem, you can just buy the proteins you want and cook in the apartment.
If you are in a tourist hotel, no problem, Germans travel to Italy in huge flocks and they tend to eat Brötchen (rolls), cold cuts, and cheese, you can just skip the rolls.
The problem will be if you are in a little pensione or a hotel that caters to Italians, breakfast is likely to be cappuccino (never ordered after about 10 a.m.) with either bread and jam, or with a cornetto (like a sugary croissant, but either filled [ripieno] with sweet almond paste, cinnamon paste, fruit paste, or Nutella, or not filled [vuoto]). If this is where you are staying you might ask if they have any yoghurt for breakfast. Italians don't eat eggs, bacon, sausage, or ham for breakfast... US breakfasts confuse them.