I would consider putting the smaller ones in the 16 gallon treat them like you would older fry. At that size they need extra coddling. Plus a separate tank helps you determine of they're eating better (poops!) and is easier to keep the food concentrated where they can see it. Because they're the same species, you should be fine plumbing the two tanks together. Is the 16g bare-bottom? If not, I'd remove the sand too; because it will make observing whether or not they eat easier.
Forgot to mention this earlier, but OR has in the past sold cold water seahorses as being safe for warmer tanks, to the detriment of the seahorses. Scores of H. capensis died because they were sold as being able to live at tropical temperatures by Ocean Rider. In short, don't trust what they say about temp.
