The size of the tank is fine for a nano tank. Invertebrates can work for you. I have kept a little tank a number of f times over the past 8 years. Do not have one at the moment. Due to travel.
If you would like to keep Dwarf seahorses, their main concern is keeping live food available for them. A very small tank is all that is needed for their basic housing. The worry with a small tank, are errors in tank keeping in a small tank usually result in big problems.
If this your first SW
tank, I would begin with the inverts option as Seahorses are very specialized in keeping needs. Require daily special feeding times and you can't really miss feedings or maintenance.
For my SW
starter tank, I used live rock and live sand and a couple of Camel Shrimp. A 5G glass Fluval Chi glass tank part only. Weekly water changes. A nano Aquarium light. Air Bubbler.
In my profile albums there are a few which are from the 5G, 12G, 6.6G and 15G.
The tank in your pic, is it specifically set up to be used primarily as a SW