As far as starfish go, it’s best to avoid the large predatory species such as the chocolate chip starfish and the beautiful red-and-white knobby African starfish (Protoreaster spp.). I would describe predatory sea stars such as these as "opportunistic omnivores," meaning that they are likely to eat any sessile or slow-moving animals that they can catch or overpower. For instance, I would not trust them with snails, clams, tunicates, soft corals and the like. Most fishes are far too fast and agile to be threatened by sea stars, but seahorses are sometimes an exception due to their sedentary lifestyle and habit of perching in one place for extended periods of time. What occasionally happens, in the confines of the aquarium, is that a predatory starfish may pin down the tail of a seahorse that was perched to the piece of coral or rock the starfish was climbing on, evert it’s stomach, and begin to digest that portion of the seahorse’s tail that is pinned beneath its body. That’s a real risk with large predatory species such as the beautiful Protoreaster starfish, which are surprisingly voracious and aggressive for an echinoderm.
But there are a number of colorful starfish that do well with seahorses. Any of the brightly colored Fromia or Linkia species would make good tankmates for seahorses. However, bear in mind that, like all echinoderms, sea stars are very sensitive to water quality and generally will not do well in a newly established aquarium. Wait until your seahorse tank is well-established and has had a chance to mature and stabilize before you try any starfish.
Two attractive species I can recommend are the Fromia Sea Star or Marbled Sea Star (Fromia monilis) and the Red Bali Starfish (Fromia milliporella), which are safe to keep seahorses. They are not nearly as delicate as the Linkia species and should do well in a standard seahorse tank that has lots of live rock and optimum water quality, and are nonaggressive starfish that feed primarily on detritus and meiofauna on live rock and sandy substrates.
Serpent starfish can sometimes be a problem under certain circumstances as well. A serpent starfish has no teeth and cannot chew; it must swallow its meals intact and in one piece, so anything that is too large for it to stuff into its oral cavity is quite safe. However, they are real stretchbellies so you have to be cautious with especially large specimens. For instance, I can tell you that when I feed my serpent starfish pieces of cubed cocktail shrimp, you can clearly see a square lump in the body disc of the seahorse for each piece of the cocktail shrimp it has ingested. Green serpent starfish, in particular, can be trouble, but if you select a fairly small serpent starfish and large seahorses such as adult Mustangs or Sunburst (Hippocampus erectus), they would probably get along together fine in a 46-gallon aquarium.
A large serpent starfish is a fascinating animal. They will hide under rocks or coral to get away from the bright light, but have an excellent sense of smell and will emerge from hiding the moment they detect anything edible, including frozen Mysis. When they are out and about, or tracking down their next meal from the tantalizing scent trail it leaves behind, they can be amazingly active and lightning fast, pulling themselves along arm over arm much more like an octopus than your ordinary, stick-in-the-mud, slowpoke sea stars. And they are excellent climbers. They pose no danger to any fishes that are too large for them to cram into their oral cavity in one piece, so there’s ordinarily no danger that they might regard your seahorses as a meal, providing you are not keeping dwarf seahorses are one of the other miniature breeds. But I certainly wouldn’t trust them with dwarf seahorses or newborn seahorses. Small, bottom-dwelling fishes such as certain gobies could be in jeopardy from a large serpent starfish, but they are primarily scavengers rather than predators.
Bear in mind that there are many different types of serpent starfish and that may also make a difference, Ricky. For example, I have found that the green serpent starfish can be particularly aggressive and I would avoid that species. I have a bright orange serpent starfish (Ophioderma squamosissimus) that has not caused any problems and the brittle stars are generally fairly benign as well.
However, it’s quite likely that any serpent starfish would attempt to monopolize the feeding station and scarf up the frozen Mysis as fast as it could stuff them into its oral cavity with its many arms. This could complicate things at feeding time for you and might become a major pain in the neck over time. Elevating the feeding station, which is a good way to thwart bristleworms and hermit crabs that are attracted by the tantalizing odor of frozen Mysis, often won’t work with the serpent starfish because they are quite agile and very accomplished climbers. But if you can overcome that obstacle, feel free to keep your serpent starfish providing it is not a green serpent starfish, Ricky. It might be a good idea to target feed the serpent and keep it well-fed, and it’s prudent to have a backup plan in mind just in case you need to find a new home for the serpent starfish if it consistently outcompete your seahorses at feeding time.
Best of luck with your seahorses, Ricky! Your best bet is to stick with the Fromia and Linkia species if you want to have a star-spangled seahorse setup.