It wouldn't be wise to attempt seahorses in a refugium that is attached to a reef system, even if the size is large enough.
Occasionally it works, but usually the seahorses will perish for one reason or another.
Seahorses are notorious for falling victim to pathogens that they get exposed to later in life, that they haven't been exposed to in their growing up time.
For that reason, best chances of success come when you don't mix seahorses with any other seahorse/pipefish/fish that has been bred somewhere else.
Doesn't mean it CAN'T be done, but the odds are against it.
Secondly, seahorses are very prone to being victimized by bacterial infestations.
As our reef tanks are normally kept quite warm, the bacteria tend to multiply much faster, in fact they increase exponentially with each rising degree.
Experience has shown that the best temperature range seems to be in the 68° to 74° range with some having no problems up to 78°.
I suspect the success at higher ranges is due to better husbandry as keeping the tank clean of any trapped detritus/uneaten food, lessens the opportunity of the bacteria to multiply.
Unfortunately most of get lazy spells, and that is when the damage may be done. Leave a little, have the temp a bit high, and all of a sudden, seahorses begin suffering.
Lastly, some people have found that many sps corals require very clean water, and with the messy eating habits of seahorses, sps losses may occur.
Seahorses are usually very picky about what they eat with less choice pieces being left to decay. Also, as they snick their food, they masticate the food and pass particulate matter into the tan via their gills, further degrading water quality.
If after reading this you still have an interest in keeping seahorses, check the links at the bottom of "
My Thoughts on Seahorse Keeping".
These links are to pages that have been written by experienced keepers and a commercial seahorse breeder to aid in the learning process.
If you have questions after that, you can ask specific things and someone is going to be there to try to help you.