Can I share a dish or ask for substitutions in Italy?

Sharing is fine and normal, especially antipasti; heavy substitutions are where you'll meet resistance. An Italian menu is structured in courses — antipasto, primo, secondo, contorno — and you're not obliged to order all of them; two courses between the two of you is completely acceptable. What gets a cool response is rewriting a dish: no cheese here, sauce on the side, chicken added to a pasta. Genuine allergies are taken seriously — say "sono allergico a…" and kitchens are careful. The distinction is between a medical need and a preference, and the second one reads as not trusting the cook.