Don't worry about the designations for restaurants, as they no longer apply. There are some very fancy Michelin-starred restaurants that call themselves "Osterie."
Similarly, as others have noted, order whatever you want. When I travel with my partner, we often choose two antipasti, and then split a pasta and split a second course. But you can do anything you like within reason.
If you each want only one course, maybe take a pasta (primi) or main course (secondi), not just two antipasti. But whatever you order, if you do it with a smile, you'll be ok..... I always engage the waiter, if possible, maybe saying something like "I have no appetite tonight so we just want one small bite...." Unless you are in a pure tourist restaurant, you should be fine and you should not be in those pure tourist places anyway..
Keep in mind that most, if not all, restaurants will allow you to order a half portion of pasta and, often, a half portion of other courses. I could never eat en entire pasta course and have room left for anything else. Always ask about this... And if you re ordering seafood or fish, it should be sold by weight. So just ask: "How many shrimp would be in 200 grams??). or: "I'd like six razor clams..." (I can never pass up a few grilled prawns if I'm near the coast!!)
I think the ordering of fish and seafood is the main issue that raises problems in Italy for foreign diners....so make sure you understand all of this. And always go to the display of seafood and ask questions if you re not sure...
You can say: "I want the smallest orata that you have tonight..how many grams would that weigh, about??"
Some restaurants also price meat, especially beef steaks, by weight.
More and more restaurants in the mid-upper price ranges now offer tasting menus. These need to be ordered by both parties at the table. I do not like them, as they are often just too many courses and too long a meal... But even if the place is listed as tasting menu only, it pays to ask if you can order a la carte.
Yes, it's usual to order bottled water and the first question the waiter will ask you, often, is: Sparkling or still. If you do not want water, you do not have to order it. You could ask for tap water but that's a little crude in my opinion....
Soft drinks are fine and, as noted, the house wine usually tastes just fine if you are not a wine fanatic. (I'm not!)