Harlequins are safe with seahorses. You probably haven't been able to find much info b/c since the harlequins need seastars for food, most SH keepers don't attempt them.
That being said, the SH still might not be happy depending on what type of nano setup you have. I wouldn't put a pair of seahorses in anything less than 29 gallons, wouldn't keep them above 74 degrees F, and wouldn't keep them with any stinging corals, anemones, urchins, or anything else that wouldn't be safe for them to wrap their tails around.