[rayjay]

Obviously the best chances of success come with a species only tank.
However, there ARE successful mixed tanks but like Dawn says, you really need to understand the risks.
Seahorses are most susceptible to bacteria problems followed by poor ability to acclimate to pathogens introduced to them by other tankmates, especially if coming from the wild like most pipefish do.
I agree that Ocean Ryder pipefish are the best chances of success for that.
While occasionally it has worked out for clownfish companions, usually it doesn't. It's not too bad when clowns are small, but they get more territorial as they mature causing stress to seahorses that often is followed by bacterial problems. More clowns equal more stress
If you have the RIGHT temperament of clown(s) and if the seahorse happens to have a better than average capability of dealing with stress, you win. Unfortunately the alternative is most often the result.
Seahorses, like humans, have varying levels of disease resistance, where some people seem to be always coming down with something while others never seem to get sick. In the fish world, most seahorses appear to be on the very worst end of susceptibility.
As for substrate, while it looks nice, it can be a detritus trap, especially the coarser the grade as it allows the smaller portions of detritus to get down into the substrate fuelling bacteria propagation.
IMO, husbandry for seahorse tanks has to be much more so than that of the fussiest reef tank because of the bacteria problems.
Now, the latest recommendations of 30g for a pair of standard seahorses with an additional 15g for each additional pair, have begun to change now that Dan Underwood of seahorsesource.com has reported that he has seen better chances of success for those going 30g for EACH pair, not just the first so that your 60g tank would be suitable for two pair of standard adult seahorses. Adding MORE livestock is going to require an even larger tank, OR, it will require a LOT more husbandry and larger water changes than a normal seahorse tank which is already more than you would do with your reef tank.
The BAD part about extra diligence is that we humans often tend to become slack in needed chores when over time, nothing wrong has occurred, but skipping a task here or there, or going a bit lighter on cleaning once in a while, means the water quality gradually degrades to the point where nasty bacteria have food and bedding to go into expansion mode leaving the more susceptible seahorses at risk.
There are NO test kits available to the hobbyist to be able to tell you when this bacteria expansion is going to happen so we need to do the preventative husbandry to try to avoid problems.
I've been keeping and breeding seahorses for 15 years now and I hate to think of how many losses I had in my earlier years just because I thought I could handle things well having been in the reefing hobby for 20.