Seahorses can deal with high flow areas as long as there are sufficient low flow and medium flow areas they can hitch in, choosing what they want at any given time.
However, it is not recommended to put seahorses in a reef tank that has warm water, corals that sting or clams that can close up on tails.
Most fish in a reef present problems in that their movements will stress the seahorse to the point it probably won't eat, and if it eats, the others will eat the food before the slow eating seahorse picks a piece he likes and eats it. They can be very picky eaters which also can present water quality issues in a reef tank.
Temperatures recommended for seahorses would be 68° to 74°F, not because seahorses can't live in warmer water, but because bad bacteria like vibriosis for instance, multiply exponentially with each degree rise in temperature, especially after 74°. Seahorse are extremely susceptible to bacterial infestations, and, are very vulnerable also, to parasites that they haven't been exposed to while growing up, but are exposed to when you put them with, or add other fish to them.
While seahorses have been kept successfully in less than ideal conditions, many many more die in the attempts to do so.