The purpose of taking niacin for your daughter is that a small amount will pass through the breast milk. Just use a very small amount, such as 50mg.
Also, you should give in to any cravings that you have, because your body probably knows what it needs better than we do. That includes sugars, fats, anything, no matter how silly it seems.
In our conversation with you, we were impressed that you are doing almost everything right. We expect for your daughter to grow up strong, healthy, and intelligent.