We are quite sure your physician has given you the supplements you need to take including a multivitamin and mineral formula, calcium and iron (if still menstruating) since gastric bypasses compromise the ability of the intestines to absorb necessary nutrients from foods. Additionally, these vitamins and minerals will be in specialized forms including chewable as needed. So stick with what the doctor's recommends.
For weight loss I would only use the meal replacements like the LeanMR, Pre/Post, and Meal Replacement powders or bars. Use them as 1 or 2 meals since you will probably be consuming 4-6 small meals. Just make sure it fits into the daily caloric allotment based on the goal.