While that is good advise for most species, it's not true for all... That tank could be made to work well for dwarf seahorses. The only problem is that you would need to keep several because that is a bit big for just a couple.
If you do decide to try dwarfs, keep in mind that the require a constant supply of live food.
I currently have a tank that is similar in size to that one, and my dwarfs are doing quite well. I think I had 8 to start with, but they breed constantly so I don't have any clue what the population is now.
Setting up the tank should have no live rock or sand (in order to avoid unwanted monsters), and should be allowed to fully cycle before ordering the ponies. They seem to like macro algae, and no other tank mates other than dwarf pipefish.