I think your setup sounds cool, I have about 4" of sand on the side with grasses, it slopes down to about 2" in the middle. If I was going to do it over I would do at least 4 all over and 6 in some places. I also think you should go with 250's in a tank that tall, but I'm no lighting expert.
But I do seahorses have several tanks of them, I'm not sure feeding even CB seahorses in a 150 is going to be very productive. They don't really "go after" food, so you are going to have to be able to target feed them, pretty tough in a 150g. I wouldn't do them in a tank that big unless you are very familiar with horses! We had our hands in the tank all the time just feeding 4 horses in a 55g, trying to make sure everyone got feed. They got moved to a 20h and things got easier. But it would be a pretty awesome tank if you get it worked out so feeding isn't a problem.