I assume you mean Dwarf Seahorse rather than Pygmy Seahorses (there is a difference; pygmies are about 1 cm tall as adults, whereas dwarves are about 1 inch tall as adults)?
Anyways, one concern I have is that while Innovative Marine Nuvos are nice tanks in general, 16g is too large for you to easily be able to concentrate the food to ensure that each seahorse is fed enough. If I were you, I would consider trying to split the tank in half so that you have two 8g sections with 4 seahorses in each section, which would make it much easier to concentrate the food.
As for being "extremely sensitive to their environment," they are no more so than your average seahorse. The problem (and likely the reason why you heard this) is that because they are fairly poor swimmers (more so than larger species of seahorses, too), and they must be fed large amounts of live food (enriched brine shrimp are the best option for this), you need to fill the tank with extremely high concentrations of food in order to ensure that they get enough to eat. Seahorses are very messy eaters in general, and having to flood the tank with food like this compounds the problem, so that whenever you feed them, your tank water will get considerably dirtier. Seahorses are more sensitive to water chemistry than other fish, so this is problematic with dwarf seahorses, and that is why frequent water changes and plenty of filtration are extremely important to keep them healthy. I could go on about this, but the articles I am recommending explain it thoroughly.
As a starting point for information on dwarf seahorses, I recommend reading the article about them on Fused Jaw (.com). Also, I believe that Seahorse.org has a "care sheet" specifically for dwarf seahorses, though you would need to create an account with them to access it.
One last thing- you mentioned that you only really want 2-4 dwarf seahorses? If you mean in total, you might want to reconsider them. Dwarf seahorses breed like rabbits (worse, even), and those 2-4 seahorses will multiply in short order to become as many as 100!