Seahorses and pipefish don't need a lot of swimming room, so the size of the tank shouldn't be a problem. But, they do have some special feeding requirements which might make keeping the water clean a challenge. Seahorses and pipefish don't have traditional digestive systems and must be fed often, with 3x a day or so being ideal. This could make keeping water chemistry in check a challenge. They also don't like strong flow, so you'll need a relatively gentle current in the tank if you want to keep them. This rules out the keeping of corals that require strong flow. You'll also need to monitor for aggression from the clownfish. Seahorses and pipefish are relatively slow, so they might get picked on by the clowns.
Personally, I would keep seahorses or pipefish in their own tank, or in a larger tank to reduce aggression and help mitigate some of the other caveats that exist with them.