Welcome
About a decade ago, I kept, and bred, Erectus and Kuda horses. Never kept Comes (Tiger Tail) horses, but from what I've read, they're similar to Kuda in size and temperament. Seahorse keeping can be a challenge... I'm glad to hear you have a successful seahorse keeper advising you already.
A 54 corner is a good size for a small herd of medium sized horses. Maybe 4 adults. I wouldn't put many more in there. You're talking about a 6" fish, and seahorses are messy eaters. Predatory, on a small scale, with a very inefficient gut. They need to eat lots, and waste lots of food. A low fish density will make it easier to maintain good water quality.
Tankmates: You mentioned Pipefish, a Diamond goby, and corals. Pipefish are wonderful seahorse tankmates, with similar life style and eating habits. The Diamond goby might be a mistake... they can be food aggressive, out-competing your horses at the trough. Corals: Many are seahorse safe, but many aren't... Goes the other way, as well. I used to keep kenya tree and colt corals with my horses, and they weren't real happy about the horses hanging on them all the time.
There's a good article and list of seahorse tankmate suitability at:
http://www.seahorse.org/library/articles/tankmates/tankmates.shtml
Raising baby horses is challenging. I was reasonably successful, back in the day, it's a lot of work. I had a bout with gas bubble disease, took out my whole herd, many years ago. I never tried horses again after that, between the heartbreak of loosing the herd, and the labor required in raising their offspring, I just figured it was time to move on.
There is not much in the aquarium world that can compare to watching a mated pair of horses performing their daily bonding dance at sunrise each day. Incredible creatures.
Enjoy
