Is it rude to ask for parmesan on seafood pasta?

It's not rude, but you'll get a pause — in Italian cooking cheese and fish don't go together, and the kitchen believes it would ruin the dish. Nobody will refuse you outright; a waiter may gently check that you're sure. The same applies to asking for cheese on a dish that doesn't come with it. This falls into the broader pattern: Italian food is regional and rule-bound because the rules are what make it consistently good, and a cook's resistance is pride in the dish rather than disdain for you. Try it as it's served first. That's the whole reason you came.