There are varying opinions on what to put in with seahorses.
IMO:
In the case of fish, while some may be compatible, you are definitely increasing the odds of transferring pathogens from the other fish to the seahorses, and seahorses DO have a problem dealing with pathogens they haven't been exposed to while growing up.
As for corals, most sps will have a problem dealing with the "dirty" water that is inherent with seahorse tanks due to their eating habits.
Generally speaking, the other corals could be considered suitable if they don't sting, and, will tolerate the temperature range of 68° to 74° recommended for seahorse keeping. Probably best to give them a dip of some kind to lessen the chances of pathogen transfer to the tank from wherever the corals come from.
Another consideration is the light requirement of the corals, as some high light demanding corals might end up with tanks too warm.