sorry, that wasn't very helpful. I was on my way out the door.
with that type of tank, I'd have a nice display of live rock and some hardy non-aggressive inverts, like some small hermit crabs (like scarlet reef hermits), snails and shrimp (like a skunk cleaner or blood shrimp). For fish, maybe an orchid dottyback and a shrimp goby with a tiger pistol shrimp.
You really can't keep a lot of fish in a tank that size. Others to consider: clown goby, pair of neon gobies, MAYBE a sixline wrasse.
What kind of filtration and lighting will you have?
My first tank was a 20g hex and I had a firefish, a rusty angel and a yellow clown goby in it for a year before I transfered to a 55g. Like Lisa said a shrimp, some hermits and a few soft corals like mushrooms and zoo's make for a nice tank.
20 gallon is a bad size for seahorses. really too big for dwarf seahorses - it's hard to get the right food density without polluting the tank (true with any dwarf tank). 20 gallon is also too small for most medium to large seahorses. it just doesn't work with most species currently available in the US.