I have to disagree with the WPG rule- i dont think it would apply in this situation. WPG isn't really a rule as such, but a general guideline. Wattage here would not be as important as the type of lighting. I would at least make sure its PC or VHO. Xenias DO need a higher amount of light.
Also- the point was raised about current-- xenias DO require higher current, i have mine in almost the 'roughest' place in my tank- the place where 3 powerheads meet, and they are thriving. Seahorse tanks generally have less current, as seahorses like lower current. I would steer clear of xenias in the seahorse tank.
But if you are not too concerend about whether or not they make it- get some for your larger tank, and see my other post about getting the xenia to grow onto another rock, then move a single xenia stalk into your seahorse tank-- then you're not killing all of them, and you can then add the rest to your seahorse tank if they do well.