20 g is way too big for dwarfs (unless you have quite a lot). I have dwarfs since 2 years.
You can easily house a hundred in a 10 g. In 20 gallons you will struggle to have enough live food suspended all day long in a good density and manage the water quality. Dwarfs are not the easiest to start with, and they are really small, much more than most people think. You need daily BBS hatching, daily BBS enriching and almost daily siphoning. It s very labor intensive and they need a constant care. Most people that try dwarfs can t keep them more than a year due to the loss of maintenance schuledule.
Pigmy seahorses are not available in the trade and the only ones that have been exported to USA are in california in a lab and it was a real pain to be able for scientists to get them due to regulations on CITES species.
20 g is way to small for other available species
It should fit a few species but they are not available in the trade (capensis, hilonis...)
So you should at least have a very minimum of 30g
Dont forge t that seahorse need to be fed 3 times minimum per day, everyday, meaning you need to find a horse sitter staying at your home even if you go away for the week-end.