Most species of snails are also perfectly safe for SH setups. Nassarius snails do a fine job of keeping food from rotting on the sand bed, and fighting conchs are a good addition as well. Both species "stir" the sand, and fighting conchs also are known to munch on diatom and some forms of cyano. Turban snails, atlantic grazers, and turbo snails are also fine for a SH tank.
Teresa is spot on regarding the hermit crabs. I've never been afraid to keep the little blue-leg hermits in my seahorse tank, but I wouldn't suggest anything larger. Most of the other sp. of hermits might be too nippy for your purposes. Emerald crabs, IMHO, are a definite no-no. Most starfish are acceptable with seahorses, but each star species has its own delicate requirements, and may not be compatible due to tank size and/or tank temperature. So-called "sand sifter" seastars aren't very good for ANY aquarium less than 200g (and even then I'd hesitate) because they have been known to eat the sandbed bare of all the beneficial bugs, etc. Peppermint shrimp are great for general scavenging and aptasia control, but you'll have to look for a larger specimen. And remember, not all peppermint shrimp eat aptasia!
Honestly, I think the best CUC in most Seahorse tanks is the tank's owner. Snails and the occasional hermit and/or shrimp crew help to some degree, but diligent maintenance and weekly water changes really go a long way to keeping a SH tank in good working order.
