The reason that height is mentioned in seahorse care guides is that seahorses need tanks at least three times their height to reproduce - their courtship dances involve a lot of vertical movement (except in one species: Hippocampus capensis). Aside from that, the only height requirement is that it is tall enough for them to be able to swim around upright, and ideally have some vertical space to maneuver in. For that reason, I would fill it up to 12 or 14 inches. (By the way, do your height measurements account for sand or not?)
The only other thing I would be concerned about is that the seahorses might get infected by a pathogen that was introduced by one of your predators (I assume that they are wild caught?). The problem with seahorses' immune systems is that they have a lot of trouble adapting to foreign pathogens once they have reached adulthood; any captive bred seahorses you buy will only have encountered and learned how to fight the kinds of pathogens found in captivity, so they will be vulnerable to any foreign pathogens that your predators brought in on them. (Ditto for pipefish, though they are perhaps a bit more hardy.) While getting wild caught or tank bred seahorses might solve this problem, they would in turn be much more difficult to keep alive than captive bred ones. None of this is necessarily a deal-breaker, but it's important to be aware of it if you do go through with this.
While the height of the tank is fine for seahorses, pipefish would have no problem with it at all. So, you could definitely get both seahorses and pipefish, just the former, or just the latter. It's your choice. As for species recommendations... well, if you get seahorses, you might look into getting Hippocampus fisheri, since they are some of the shortest captive bred species available. On the other hand, in a tank that large, you might not ever be able to find them. In terms of hardiness, your best bet would be Hippocampus erectus, which, coincidentally, are also the easiest to obtain. Hippocampus reidi are also hardy, but they are among the taller species of seahorses, so it might be better to avoid them. Hippocampus kuda would also be fine; they are not as common on the market as H. reidi or H. erectus, but ORA, Seahorse Source, and Ocean Rider breed them, and I think that Live Aquaria now stocks ORA H. kuda as well as the wild caught ones. As for pipefish, I suggest you take a look at Fused Jaw(.com): they have a series of articles on the subject of keeping pipefish, including an overview of the species better suited/more available for aquaria. Also, I should mention that Ocean Rider now offers a species of pipefish, making them the first and only ones to regularly breed and sell a pipefish species.
The only other thing I am concerned about is the flow rate. Now, for the record, I have no idea whether or not 550 gph is too much for seahorses. GPH is not an accurate indication of the speed of the current in the tank, so it may be just fine - there is no way to tell from that number alone. However, since we really can't say whether or not 550 gph is appropriate for seahorses, I suggest you make sure that there is a "peaceful" area of the tank by, for example, using live rocks to block off a corner (not completely, since you need to leave space for the seahorses to enter it, but sort of like how people use those really tall trees as windbreakers). Seahorses can deal with much more current than most people assume, but since they use only their one little fin to swim around, it is important that they have a low-flow area to which they can retreat when they are tired out. So as long as you make sure that they have somewhere sheltered to retreat when they get tired, the current - whatever it is - should be just fine.
