Most soft corals are considered safe tankmates, although some soft corals can damage other soft corals. The ones to avoid are the non-photosynthetic ones, not because they have potent stings, but because they are likely to die in captive systems, and having something decaying in the water really destroys your water quality. There is one exception to this that I can think of, and that is the elephant ear mushroom, which can, and will, eat fish. Seahorses have few resources to keep them from being consumed, so I wouldn't chance tanking one with a piscivore.
Seahorse keepers really vary in what they feel comfortable with, in regard to tankmates. I won't say that any particular way is correct, although I do feel that some ways are wrong! Basically, you want to make sure that your seahorses are safe. In regard to any fish tankmates for your seahorses, here are just some of the things you need to consider:
How aggressive is the fish? Obviously, seahorses don't have any way to spar with a tankmate, so they should not be housed with fish that are going to act hostile towards their mere presence.
How "fleet of fin" is the fish? Although most owners will caution that seahorse tanks need to be set up with low flow areas and such, many seahorses like to explore all areas of their tanks, although they may only do so for a specific hour each day. Very active fish dashing around the tank will increase your seahorse's desire to stay hitched and secure, and will interupt pair bonding.
How gluttonous is the fish? Seahorses have very inefficient digestive systems and need to eat frequently. They also have a tendency to study their food before they strike and "snick" it down. Gluttonous fish that seem to constantly be ravenous for food will make feeding your seahorses difficult, and a poorly nuriushed seahorse can quickly become an ill one.
What are the natural behaviors of the fish? Seahorses can hitch to objects for long periods of time and are known to have algae grow on them. Fish that feed by rasping algae or that enjoy picking at rocks/substrate/etc. can both annoy and harm seahorses.
All of that said, even if I had all of the tanks that I would need to house all of the fish that I would like to own, I would likely still elect to house additional fish species with my seahorses. I simply think that the list of truly suitable and safe fish tankmates is pretty small, unfortunately.
