I understand what you are saying regarding the size of tank preferred for dwarfs. You are correct in this. What the others are trying to say (I think) is that saltwater
tanks have very unstable water parameters in small tanks. It is highly recommended that people new to saltwater start with the largest tank that they can afford to maintain. Then, after learning to recognize the signs of things going right/wrong with the tanks, getting good experience, then move on to some of the more challenging small tanks. Even though you read about the needs of seahorses, which incidently, I commend you highly for doing, you cannot begin to fathom the sensitivity of these creatures. I speak from experience and they are not for the beginner.
If you choose to continue with this venture, be sure to observe your rock under good lighting for a couple of weeks AFTER the cycle. Seahorses can be killed by rock dwelling things such as aiptaisia, hydroids, stinging corals and small crabs.
You want to make sure your rock does not contain any of this and only by observation will this occur. You mention plants, were these to be live macro? or plastic? If it is live, you will need to learn to prune efficiently or some varieties of the macro can crash your whole tank, taking all life with it, within just a matter of hours.