Yes, Mustangs and Sunbursts are different color morphs of the same species (Hippocampus erectus), so of course they have identical aquarium requirements and make wonderful tankmates. Sunbursts are very similar to Mustangs in most respects, including their hardiness, and will even interbreed with them freely; the main difference is that the Sunbursts tend to be even more brightly colored, as their name implies. They are predisposed to display sunset colors (shades of yellow, gold, and orange) when conditions are to their liking.
However, the last thing you should do when you are battling ammonia and nitrite spikes in a new aquarium that has not completely cycled is to add more livestock, mom. I would suggest waiting at least a few more months before you consider adding any more seahorses.
This would allow time for your new 55-gallon aquarium to cycle completely and build up a large enough population of beneficial nitrifying bacteria to safely accommodate the additional wastes produced by more seahorses. It’s important to allow your biological filtration to become fully established and to give the new tank a chance to stabilize and break in before you increase the bioload any further.
More importantly, that will give you a few more months at least to learn the ropes and gaine some valuable firsthand experience with your Mustangs before you consider increasing the herd. As a beginner, it’s always best to be patient and go slowly so that you can master a few simple tasks like feeding the seahorses properly and maintaining optimum water quality at all times before you get too ambitious
If Max and Ruby are still thriving several months from now, and your new 55-gallon tank has stabilized and is keeping all of the water quality parameters right where you want them, then you might consider adding a new pair of Sunbursts. Your tank is certainly large enough to safely accommodate more seahorses once your learning curve has flattened out a bit, mom. But give yourself a chance to work out a sensible aquarium maintenance schedule, including a diligent regimen of regular partial water changes, so that you can maintain optimum water quality at all times before you commit to any more ponies.
Best of luck with Max and Ruby, mom!