One of the problems with hatching brine shrimp is that they like it cooler than most aquariums. I've heard of people hatching them in a refrigerator, but I don't know how you get the air pump to run inside a fridge! One problem with putting them directly in your tank is that the eggs will float up to the top and create a mess. The eggs float, so you'll end up skimming all of the eggs off anyway. Also, it is unlikely you'll be able to keep a population growing in a tank together with something that will be eating them. You have to have an adult population to produce eggs. Another thing is that brine shrimp are not enough to sustain seahorses for very long. They are so tiny when they hatch.
Good luck,
Sonny