As far as starfish go, it’s best to avoid a large predatory species such as chocolate chip starfish and African red knob 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 are the popular chocolate chip stars, which are surprisingly voracious and aggressive for an echinoderms.
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 the tank such as you’re planning 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.
In short, Sean, your Bali starfish should not cause a problem for seahorses. It’s a small species that doesn’t grow to more than 3 inches in diameter (most aquarium specimens are only 1-2 inches in arm span). They thrive in a well-established aquarium with lots of live rock and a sand substrate.
Best of luck with your starfish, sir! I’m sorry to hear that the sea star had a misadventure with your anemone.