All seahorses have their own distinctive personalities. Some are naturally shy and retiring, whereas others are more curious, bold and outgoing, just as people can be considered either introverted or extroverted. So if you obtained more seahorses, there is no guarantee that the newcomers will be brazen and aggressive compared to your present pair of ponies.
But I can tell you that Ocean Rider seahorses are very social, highly gregarious animals that ordinarily very much enjoyed a copy of others of their kind. They are also accustomed to the presence of people, and generally seem to enjoy interacting with their keepers. For example, they are typically friendly and quickly learn to recognize their feeder as the "giver of gourmet delights," whereupon they will often include their keeper in their daily greetings and social interactions.
And as long as you avoid overcrowding, adding some fresh blood will almost always trigger a flurry of renewed activity and greetings as the seahorses reassess their shifting social dynamics and check out their prospective new partners. As an added bonus, the newcomers increase the genetic diversity of your herd and help prevent inbreeding, strengthening your seahorse’s gene pool.
So I think that introducing a new pair of seahorses to Fix and Foxi may well liven things up a bit. I think it’s a sensible plan to wait until Christmas and gain little more experience with your seahorses to make sure you’ve got everything down pat before you consider expanding the herd. If all is going well at that time, then you can confidently add another pair to keep Fix and Foxi company.
Best of luck with all of your seahorses and their offspring, Christine!