It's not that your body is unaccustomed to fat, it's that it can't process it. When you eat fat, your gall bladder releases just enough bile to break the fat down. When you don't have a gall bladder, you get several problems going on. You don't have enough bile to mix with your fat when you eat it and so it doesn't get broken down properly--and can cause diarrhea. And you don't have a little sac to store yoru bile in, so it drips from your liver constantly and you can end up with too much bile in your intestines when there's no food in there to absorb it and thus, diarrhea. It's basically a timing problem now with not getting the bile when and where you need it in the right amounts. You can try never eating anything fatty again, but I have found that's impossible to accomplish. The best I made it was 4 months before I went crazy for some pizza. Like any extremeist diet, people just can't stick to it. And I found that going too low fat made me constipated. That's a normal reaction for normal people, but was even worse for me because my guts aren't properly balanced to begin with.
The best thing to do is practice moderation. I take my Welchol so that I can eat normal foods with normal amounts of fat, but I pick and chose my fatty foods. For instance, I never eat fast food except when we are traveling, and even then I try to avoid it. I almost never eat any fried meats anymore. I very rarely eat out and really can't tolerate doing it more than once a week anyways. Gravy and biscuits is right out. I only occasionally eat sausage and eggs for breakfast since that tends to sit too heavy on my stomach in the mornings. And I can't have any caffeine except in a mug of herbal tea; that was my trigger before and still is and will make me sick even with my regular dose of Welchol. But I can eat pizza (Red Baron suits me, as do deli pizzas, but pizzas from PIzza Hut, CiCi's, etc. are too greasy) and once in a blue moon I can have some of my grandmother's fried chicken. I can tolerate Chick-fil-A because they use peanut oil which is a good fat, unlike the partially-hydrogenated crap everyone else uses. And I eat a little cheese most days and can have a cup of creamy soup for lunch and make French recipes for supper with cream and butter. Fat doesn't bother me so much as caffeine and grease. Everyone is different, so you will have to find out what things you can have and what you can't have.
Calcium carbonate is what you have to have. But I'm thinking Tums doesn't have a lot in it? I take one 600mg calcium pill a day, but you can take up to two. The other reason why Caltrate 600 w/ vitamin D specifially is recommended is because it has a low amount of magnesium in it. Since calcium carbonate is constipating and normal people don't want to be constipated, they add magnesium into most calcium supplements in order to get them balanced. The Caltrate 600, according to Inape (Linda) is the best kind to use because it's the most constipating. Other formulas won't work as well or not at all.
I have gotten pain with my bile diarrhea before, but the calcium really helped it. I think that the excess bile irritates the intestines and causes the pain. Calcium can both help absorb the bile and it helps soothe intestinal irritations. As far as I am aware, though, the bile will not permanently harm your intestines.
I don't know if Pepto will help the cramps, but getting the diarrhea stopped usually eases or stops them. You can always take an OTC pain pill (not aspirin) to help with the pain. Heating pads or hot water bottles also can help.
It is my understanding that your body will take 6-8 weeks to adjust to not having a gall bladder. After that time, what you have in terms of reactions and bowel movements is pretty well what you can look forward to from now one. So your doctor may be reluctant to prescribe you something before that period of time is up, but, personally, I would stay on him about the Welchol. You don't have to take it when you don't need it--in fact, it can be constipating, so you shouldn't take it when you don't need it. But you can take it when you are about to eat something you know will bother you or you can take one if yoru guts get stirred up; it works better than either imodium or Pepto in getting your guts settled back down. Or, you may be like me, that, in order to eat a normal diet, you have to take one or more every day as a matter of routine.