As a data point, we keep our seahorses and pipe-horses in 40 gallon hex that is plumbed into our main reef system.
Our banded pipefish (or dragon-faced pipefish) do just fine in the big mixed reef (note - it's a pod making monster, they've never transitioned to frozen/dry food, and have been happy and growing for a long time)
There's no way the seahorses could make it in the main tank due to flow and competition for food.
If you're careful about species selection related to temps, you can run the seahorse-species tank plumbed into your reef, and gain all the benefits of your exising sump/skimmer and water volume, and thus water stability. A concept that in my opinion you don't see supported enough compared to the "they always require a completely seperate system" answer.