Have to agree with the posts above - the only seahorse that will live in a 10L tank is a dwarf seahorse, they are incredibly difficult to get in South Africa (I can write a book as to why) and even then you can not keep more than 4 at the max.
They will not except frozen food and because of the high oxygen usage and spiking water level quality caused by live Brine shrimp used as food you have your hands full. Water movement is a big problem and although they are amazing they are by far the least active Seahorse in the world.
Having said the guys I do have breed well during the summer month and on the odd occasion some become available.
I am going to suggest that you follow the recommended rule, start with a large tank, nice filtration and very few Seahorses (1 or 2), probably the easiest way to overcome the learning curve and safe yourself a stack load of money.
I will see if I can upload some photo's of the dwarfs during the morning