I was instructed I could have sips of flat soda after the surgery. Later, gassy soda okay, but not desirable - because of its distention (stretching) qualities (the gas).
Further I was instructed to never drink with meals, because of the interference with the digestive juices required to break up the food and which is added after one eats - don't put additional volume in the small pouch. There is the worry of enough nourishment and then again, enough Water not to become dehydrated.
So, water at least 1-2 hours after eating was recommended, but I did drink diet sodas with no problem. I still have pain if I drink much during a meal, especially if I eat meat (too much going on in the small pouch to add a liquid-too much stretch). I suggest drink your diet sodas in between your meals and you should have no problems.