Dwarf seahorses only get to about an inch (give or take) in size so putting them in a large tank means you'll hardly ever, if ever, see them.
You could probably put around 3 dwarfs on a silver dollar.
Even just one or two in a 5g tank would make viewing hard, but they won't have any trouble living in it.
Also, because dwarfs don't usually hunt down their food, the food density of live enriched brine nauplii in the tank has to be enormous so that they can snick the food as it comes by them without them leaving their hitch. Then, you need to remove the brine remaining before the next feeding as nutrient levels would now be virtually depleted in the nauplii.
Using a large tank means a HECK of a pile of nauplii each feeding.
Unfortunately there are no other seahorses that you can keep in a 5g tank for that long.
What else were you wanting to put in the 55g? Seahorses have specific needs AND they don't do well most times when mixed with other fish and indeed, other species or same species of seahorses that are bred in different facilities.
