Erik, Welcome
Bullitr,
I'd ditch the 175w MH too. If you need the shimmer you can get away with a 70w, will be less heat and less intense. T5's are a great option to, but if you get a 30" tank like the 30g tall, then you will need to run the 24" T5's as they are currently not making a 30" HOT5 bulb as far as I know.
There is no hardy dwarf yellow seahorse . . . It's hard to sell seahorses by color, since they can change there color on a whim.
A dwarf seahorse is fairly small, around an inch, and requires special care. They would not do well in the tank you described.
Reidi and Kuda are both often sold as "yellow seahorse" and aquacultured speciments are available through seahorsesource.com