I am happy to hear that your new seahorses are big and beautiful and seem to be settling in nicely. It sounds like you did a good job of acclimating them to their new home.
Yes, that’s right — it’s best to leave the aquarium light off for the first day and give the new arrivals plenty of peace and quiet while they are making the adjustment to their strange new surroundings. Tomorrow you can turn the aquarium light on, as usual, and try feeding your new acquisitions for the first time.
Yup, when you are sexing seahorses, there is indeed a noticeable difference in the profile of the abdomen. In females, the abdomen curves inward more sharply, so that the base of the belly almost forms a right angle to the tail. In males, the abdomen slopes inward toward the tail more gently, giving the base of the belly a more rounded appearance in profile. It is the presence of the pouch or marsupium in the males that accounts for the difference in their silhouettes.
Sexing adult, fully mature seahorses in breeding condition is therefore normally simple and straightforward — the males have a brood pouch slung under their abdomens at the base of their tails and the females do not. This difference will be most obvious when the seahorses are courting and breeding, since the stallions perform vigorous pouch displays in which they inflate their pouches with water to the bursting point, making them unmistakable.
Best of luck with Max and Ruby, mom!