Whilst i like the idea of running a reef tank with seahorses in it, its certainly not something i would recomend...You dont say how much experience you have, but, after running a system like this, i would "hope" your an experienced reef keeper...
I dont think anybody will / can say that you "cant" keep seahorses in a reef tank, its just that, in my opinion, you shouldn't...Main reason, stock and flow...Most of the stock in the average reef tank would damage the seahorses, so, i would not advise that....Also, flow...Reef tanks are high in flow, average about 25 x tank volume per hour....that, in itself, makes the habitat hard for seahorses...I feel that the suggestion of having a gap like area for a seahorse is not a valid route..If we think about it, what use is this, as the seahorses can only inhabit a small portion of the tank....This in itself does not this good for seahorses....what life would they have, when they can only stay in one area...
On the whole, this is a bad idea...Just my opinion of course... :grinyes: