First, there is no need for special lighting except for what other life forms you have in the tank as seahorses have no real need beyond ambient lighting.
The reason for minimum tank sizes arises from problems specific to seahorses.
In addition to falling victim to pathogens from other seahorses/pipefish that have not been bred and raised in the same breeding facility, they are VERY prone to being affected by nasty bacteria like vibriosis which is always present in the tanks. The bacteria multiply rapidly as they need only food, and, as temperatures rise, they multiply exponentially with each rising degree.
We attempt to keep temperatures of the tanks between 68° and 74° to limit the rate of growth of the bacteria, and we attempt to also limit the concentration of food for the bacteria to feed on.
The biggest sources of food for bacteria in seahorse tanks are from uneaten food due to seahorses being selective about the pieces they eat, leaving "inferior" looking pieces to rot and decay if not removed regularly.
Also, when they snick the food, they masticate the food and particulate matter is expelled through the gills into the water, again producing food for bacteria.
By having larger water volumes, the tank is more forgiving so it allows you to keep on top of husbandry more to lessen the chances of problems.
Many have problems even with using required tank sizes because we tend to get lazy when every thing is going good, and not realizing the problem is unseen but building up during a lazy stage, until all of a sudden, something like tail rot, or flesh erosion happens.
Keeping them in a tank that is probably less than half of what it should be means houskeeping is going to have to be VERY EXCEPTIONAL with no LAZY stages. It can be done, but not everybody has the fortitude to stick at it for the long term to be successful.
