I would say that that is an exception to the rule. Not the standard. This is quoted from
seahorse.org
The seahorse tank
Before you buy a seahorse, be sure you understand the basic principles of how to
keep seahorses in the home aquarium. The best overall source for this knowledge
is Seahorse.org. Keeping marine fish of any type requires knowledge of basic
marine aquarium keeping and water chemistry. There are many books and other
sources of information available. If you prepare adequately and set up an
appropriate sized, fully cycled, and stable tank, you will greatly improve your
chances of success. Seahorses need “hitching posts”—something to cling to while
they are resting. Not having these resting places is stressful for a seahorse.
Seahorses should be introduced into a mature, cycled aquarium. Various filtration
methods and tank set-ups can result in a healthy, stable aquarium. A seahorse
tank must have gentle to moderate currents, with 3–5 times tank volume turned
over per hour. A seahorse tank needs to be void of intense currents and requires
lower flow areas where a seahorse can retreat and rest Water parameters should
be stable before animals are added:
pH – 8.0 to 8.3
Specific gravity – 1.020 to 1.024
Ammonia – 0
Nitrite – 0
Nitrate – <20 ppm
Optimum temperature is dependent on whether the seahorse species is tropical,
subtropical or temperate. Generally, beginners should start with tropical species as
heating a tank is much less expensive than cooling one, and it is easier to
maintain a stable temperature in a tropical tank. Try to keep to the lower end of the
temperature ranges, and let the temperature fluctuate up towards the higher
values of the temperature range. Taller tanks are preferred. Seahorses need
height (2.5 to 3 times the uncurled length of the animals) in their tanks to court and
mate. As a minimum, the internal height of the tank, excluding the substrate,
should be at least 2 times the uncurled length of the seahorse you are keeping.
Temperature and stocking density table for common seahorse
species
The recommended minimum tank size per pair of adult seahorses, as well as the
‘additional’ tank space required by each subsequent pair, is listed in the table
below. For example, H. erectus, allow the first pair 29 gallons then 15 gallons per
pair thereafter.
1) Tropical, 71-74 degrees F (21-23 degrees C)^
H. angustus 1 pair/15 gallons (55 litres); minimum tank size 29 gallons (110 litres)
H. barbouri 1 pair/15 gallons (55 litres); minimum tank size 29 gallons (110 litres)
H. comes 1 pair/15 gallons (55 litres); minimum tank size 29 gallons (110 litres)
H. erectus 1 pair/15 gallons (55 litres); minimum tank size 29 gallons (110 litres)
H. fuscus 1 pair/10 gallons (38 litres); minimum tank size 20 gallons (76 litres)
H. kelloggi 1 pair/15 gallons (55 litres); minimum tank size 29 gallons (110 litres)
H. kuda 1 pair/15 gallons (55 litres); minimum tank size 29 gallons (110 litres)
H. procerus 1 pair/10 gallons (38 litres); minimum tank size 20 gallons (76 litres)
H. reidi 1 pair/15 gallons (55 litres); minimum tank size 29 gallons (110 litres)
H. subelongatus 1 pair/30 gallons (114itres); minimum tank size 65 gallons (246 litres)
H. zosterae* 1 pair/1.5 gallons (6 litres); minimum tank size 5 gallons (20 litres)
^ 25 degrees C is the maximum temperature for tropical species and should
not exceed this temperature.
* Recommended to keep in groups of 3 pairs.
2) Subtropical, 67-70 degrees F (19-21 degrees C)
H. ingens 1 pair/30 gallons (114 litres); minimum tank size 65 gallons (246 litres)
H. tuberculatus 1 pair/10 gallons (38 litres); minimum tank size 20 gallons (76 litres)
H. whitei 1 pair/10 gallons (38 litres); minimum tank size 20 gallons (76 litres)
3) Temperate, 64–66 degrees F (18-19 degrees C)
H. abdominalis 1 pair/30 gallons (114 litres); minimum tank size 65 gallons (246 litres)
H. breviceps 1 pair/10 gallons (20 litres); minimum tank size 20 gallons (40 litres)
H. capensis# 1 pair/10 gallons (20 litres); minimum tank size 20 gallons (40 litres)
#H. capensis prefer wide tanks for courtship rather than tall tanks
As you can see
H. zosterae is the only compatible pony for a pico however they are recommended to be kept in a group of at least 6... so its up to you, I would not do it.
