One of the main reasons most don't keep other fish with their seahorses- other than the issue of finding peaceful enough fish- is the increased risk of introducing pathogens, etc from the wild which the seahorse's immune systems are unfamiliar with and are thus very susceptible to. The way to minimize the risk, is, of course, to get captive bred tankmates for captive bred seahorses. So I would suggest doing just that, and fortunately, there are several seahorse-safe fish which are readily available captive bred. For example, ORA breeds all three known species of Assessor Basslets, which, according to seahorse.org, are seahorse safe (except Randall's, which hasn't yet been attempted). Bangaii Cardinals are also safe tankmates, and they are fairly easy to find captive bred, as well. For someone who has a large enough tank, Ocean Rider now has captive bred Banded Pipefish. I expect the list of readily available captive bred seahorse tankmates will only increase over time, given the current push towards captive breeding programs.
