How small is "small"? If the tangs ate one shrimp, they'll eat bigger shrimp just the same. While tangs aren't typical shrimp eaters, you clearly have at least one with the taste for them. I had a royal gramma that ate full size peppermint - not typical, but no shrimp in that tank until he died. And you've got big, healthy tangs to tackle another shrimp no problem. Adding at night only means the shrimp may last a day or two before becoming a meal.