not all that tall when you consider that readily available horses that are captive bred, the largest gets at most 12 inches. 36" tall tank is pretty tall, but commonly the horses range from 4-8 inches. Can't really plan on the 4 inch side, so 24 inches is ideal. Most tall or extra tall tanks are that deep in the smaller sizes. 2.5 times the adult height isn't ideal but it isn't going to stress them to mass amounts.
That's one reason why you really have to plan ahead for a specific seahorse tank.
Wow, 3X the height of the seahorse.
How long do the pixies/pygmy live?
It would be super easy to put a sponge filter on the outlet and inlet of the tank, to prevent them from going down into the reef tank.
Thinking about brine shrimp and pygmy seahorses now... Going to do some more reading. Also going to ask around at my LFS to see what they recommend, they had a seahorse tank for a good 6 months.
I may be able to do a tall tank, but it would require me to revamp plans quite a bit...

I'm limited to a rather small space for this, I think 10 gallons is about max unless I can find a cheap 29 XXTall tank that has the same footprint as a 10 gallon tank.
I also have a tank divider for the 10 gallon tank. I could use it to turn the 10 into a 4 and 6 gallon tanks, put the little seahorses on one side and something else on the other side of the tank.
Regardless of what I decide to do, this tank will be getting set up for a good week for leak testing, then another week for pump testing and algae/'pod growth. I'll update this as I get closer to a decision. I'll try breeding the brine shrimp for a week daily before I commit to buying a seahorse. I want to make sure I can feed the buggers consistently, daily.