My experience in the subject of seahorse tankmates is just this:
Seahorses do have specific requirements for each different type of seahorse.
Research seahorses first, just as with any fish you want to keep. Know how to set your tank up, water flow, filtration, habitat, what foods. Pretty basic stuff to consider.
GOOGLE Seahorse ID and have a look. (I hope that's OK)
Plus, when you do your research you can go to a seahorse stock source and sound confident.
If you want your seahorses to thrive, be sure you know what kind of SH they are before you buy them (kuda, reidi, erectus, dwarf, pigmy).
Your tank should be set up with the specific SH species in mind.
Don't ever buy a SH that says "Black Seahorse $29.99" unless you can identify it and know if it is tropical, subtropical...captive bred or wild caught.
Next, ask other seahorse keepers experience on tankmates. Like Darthtang, says, the tankmate choices depend entirely on the SH that you choose.
The choices when you know what each fish needs are not vastly different with SH in the mix than with any other fish.