Okay, that sounds good! It sounds like you’ve already done an excellent job of aquascaping your seahorse tank to create an elaborate environment with lots of microhabitats and plenty of hitching posts, sight barriers, and hiding places for when the seahorses feel a need for a little privacy. Well done!
Yes, certainly in an aquarium that’s 30 inches tall (that’s a great height for a seahorse setup, by the way) you can include macros that extend into the water column 15 inches or more. That’s perfectly acceptable and you needn’t worry that it would encourage the seahorses to perch too high up in the aquarium where they would be more susceptible to certain forms of gas bubble syndrome or other depth related problems. Seahorses almost never perch at the very tip top of the macroalgae, so even if you included some plants that extended a good 15 inches up from the substrate, when the seahorses perch on them, it will be at least a little lower down so that you should always have a good 20 inches of water above them nonetheless. That should not be a cause for concern in the least.
Seahorses do occasionally have a problem snicking up foreign objects inadvertently when they are feeding off of the bottom. Any potential problems in that regard can be minimized by using a substrate in your seahorse tank that consists of pebbles that are too large to be snicked up through the tube snout or by using very fine sand that the seahorses have no trouble at all expelling afterwards should any of it be accidentally ingested. It is the very small pebbles that are small enough to be sucked up through the narrow diameter of the tube snout, but large enough that they may be difficult to expel again without causing abrasions or irritation that are sometimes problematic. As long as fragments of pebbles that could be potentially troublesome are relatively scarce in the substrate, then all should be well. You can further minimize the chance of foreign objects being accidentally ingested by teaching your seahorses to eat from a feeding station which you have elevated above the substrate to keep it out of reach of bottom scavengers such as bristleworms, Nassarius snails, or microhermit crabs.
I wrote an article in Conscientious Aquarist explaining exactly how to set up a feeding station and train your seahorses to use a feeder in some detail that you may find to be useful in that regard, Tammy.. It discusses all the different kinds of feeding stations, including natural feeding stations. It’s available online at the following URL:
Click here: Seahorse Feeders
In short, Tammy, I don’t think you need to worry about the little rocks in the CaribSea sand as long as they are not too numerous and you either set up a feeding station for the seahorses or target feed them in an area clear of these pebbles. If you are concerned about them you can always sift out some of the little rocks from the fine sand to be ultrasafe, but that’s probably not necessary.
Yes, pipefish make fine companions for seahorses, and you may certainly consider adding a couple of pipefish to your seahorse tank as tankmates if you like. Of course, captive-bred-and-raised pipefish would be ideal, but I know of no one who is culturing them at this time. Wild-caught pipefish are also a possibility providing you make sure they are eating before you purchase them and you quarantine them before you introduce them to the main tank.
Oh, yes — I can assure you that a community consisting of four Sunbursts (Hippocampus erectus), two "Nemo" clownfish (Amphiprion occelaris), and two pipefish will not overtax the biofiltration or overcrowd your 85-gallon aquarium by any means. In fact, that would allow you plenty of room to safely increase your herd of seahorses or add additional tankmates in the future. If you want to consider only specimens that you have purchased from the Ocean Rider High-Health aquaculture facility so you know that they are safe, then you could also consider adding a Banggai or Banner cardinals (Pterapogon kauderni) and/or a beautiful orchid dottyback (Pseudochromis friedmani) to your community in addition to other types of seahorses.
Best of luck installing your cleanup crew and getting your macros off to a good start, Tammy!