I agree with pledo that with EXCELLENT filtration (including a large sump) and no fish tankmates, that you could squeeze two pairs of seahorses in a 29, but for most people I would only recommend 1 pair, especially if you don't have experience with the bioload and nitrate problems that seahorses present. His suggestions on species are good ones.
Or, you could go with pipefish, but I would not put both seahorses and pipefish, just one or the other.