You will start off on clear liquids and then progress to a full liquid diet (which consists of strained cream soups, sugar-free pudding, plain or cream yogurt, and any sugar free liquid with the exception of carbonated beverages. You can also have fruit juices that have no sugar added). At about six weeks, my diet plan allows me to start eating a regular diet and progressively adding new foods. I developed lactose intolerance since having surgery and I have basically educated myself on how to deal with it. Right now I have an ulcer, so I am having to live on the full liquid diet for up to three more months, even though I have hit the 6-wks post-op this week, so I would be in major trouble if I didn't know what to do. I live on yogurt and sf pudding. I cannot handle anymore cream soup, so I just dont ever get anything warm anymore.

But there are lactaid drops you can buy to mix in food or milk or you can do like I do and chew Lactaid before each time I am going to have a dairy product. You can buy the chewables at Wal-Mart. I also buy Land-O-Lakes Dairy Ease Lactose-free 2% milk to mix with my protein mix, so I feel like I am getting chocolate milk alot because I love the chocolate protein mix. I also have the vanilla and strawberry, but is not as good to me. I basically cannot do cheese at all (when I was getting to eat pureed foods). It still hurt even if I chewed Lactaid before eating it. You will have to just use trial and error to see which foods you can tolerate after surgery. Some things you can eat now may be a no-no for your pouch when you are post-op and vice versa.
The reason you can't drink with meals is because liquids do not stay in your pouch for very long and if you drink while you eat, it will flush your food from your pouch prematurely and you will not feel full and will eat more and also it can cause dumping with some foods by the food going into the intestines too soon. My dr. says I am not to drink 30 mins. before or 30 mins. after a meal.
I'm sure your nutritionalist will go over the whole food plan with you before surgery so you are prepared and then you will have scheduled visits afterwards too that are very important to work out any kinks or problems in your plan. I have a wonderful nutritionalist. I have his email and I can ask him anything anytime I want and he gets back to me within no time.
I hope this helps some. Maybe some other people can add some things to help also.