Just came across an article on pepto bismol for dogs:
Pepto bismol
They recommend only 1 teaspoon per 20 pounds of weight. For a tiny puppy of only 2.7 pounds, that work out to a few drops, so I definitely wouldn't risk doing it. Kaopectate would be safer, but for diarrhea, many people on Yorkietalk recommend canned pumpkin (1 tablespoon of the pure stuff, with no sugar added).