The probiotic VSL3 is like a miracle for my IBS. There is a website for it and you can get it from your doc at double strength (start slow with prescription strength). It has lessened my symptoms by 95%. Its a vert potent probiotic with lots of testing and research done which you can research online. It's been a godsend for me.
The daily dose of magnesium is 400mgm and is best if you take it in divided doses 3 times daily. Calcium is not absorbed without magnesium and Vitamin D. At this small dose it should not cause diarrhea.
The best mineral supplements I have found are chelated that is bonded to a protein amino acid usually glycine or lysine by Albion Labs. It is a smaller size for better absorption through the intestine walls.