Mandarins and seahorses alike, do not buy one unless the LFS can show you in the store that it is eating frozen food. If you put a mandarin in a new 14 gallon tank, and it's not already eating frozen food, it will starve to death. If you have one that's already eating frozen food, you are good to go. So in a tank that small, I would not recommend you try and train it to eat frozen, but instead just buy one that's eating frozen already.
Others are right about the seahorses, they can be challenging to keep because they require low water temperatures and are very susceptible to disease. If you do decide to get seahorses, be sure to only buy ones that are tank bred. Never buy wild caught.