It’s great to hear that your new Mustangs seem to be thriving in your 55-gallon aquarium despite the ammonia spikes and have such good appetites. It’s a very good sign that they are already displaying a healthy interest in courtship and mating after getting off to a rough start.
Yes, if the seahorses are eating 2-4 of the frozen Mysis enriched with Vibrance three times a day, that is a sensible feeding regimen for them. Each of the seahorses should be eating a total of between 5-14 of the large frozen Mysis daily and then fast for one day a week.
Be sure to monitor the ammonia, nitrite, and nitrate levels in your 55-gallon aquarium regularly while it completes the cycling process, and don’t forget to study up on the books I recommended at your earliest convenience.
Best of luck with your new seahorses, mom!