First:
There was a very important param missing in your list -- the temp of that 10 gallon tank. What is the tank's temp running at? Temp can affect important things, like the oxygen carrying capacity of your water. What type of filtration and water movement is on the tank? Is there anywhere for these seahorses to hitch?
I wouldn't dismiss your low pH, either. When did you test the pH? Was it after the lights had been off for a few hours, or after they had been on for several hours? A 7.8 might be okay if it was the lowest point during a 24 hour cycle, but not if it was the highest the pH ever reached in that tank.
I think it would be difficult for anyone to assess the stress level of these horses without first knowing the above, along w/ the dimensions of this 10 gallon tank and seeing photos of the horses in question.
B:
Tom has experience w/ numerous seahorses, and he has seen horses that darken in response to stress. Scott probably has more current exprience w/ a particular species of horse than many do w/ that same species, and his observations are that these horses' colors are more a response to camouflage than to show well-being. Ray has worked hard for success raising difficult pelagic horse fry, and both he and Tim have successfully kept different species of horses. Each person's experience w/ their horses is a bit different, in part because each person's species are different, their source for stock has been different, and their methods of keeping are different.
Humans traditionally haven't thought of "animals" as being "different" from one another -- having preferences and habits. Most people certainly do not like to think that animals have personalities. But consider how much of your personality includes your "preferences" and "habits." I have no idea how far down the chain of animal-kind these traits go, but IMO fish show "personalities." That is why people tell you to observe your tank -- so you get to know what is "usual" for your tank, so you'll notice when one of your animals is telling you that there is something wrong.
So, perhaps the questions of colors are not only affected by species, environment, and stress, but by "personality" as well. After living in the relative safety of captivity for some time, a horse may modify its habits somewhat. Or its habits may be over-run by the influence of age over species-specific traits (perhaps juvies are more likely to try to blend in w/ surrounds than the adults?)
Third:
However, I think that what the people who have been posting to this thread have been trying to show your, recently, is that, beyond the hypothetical musings of horses and their color, one important thing has come to light in this thread: some veteran seahorse keepers are trying to help you be successful w/ your seahorses by recommending that you use your 10 gallon tank as a QT tank only. As for your upgrade, IME, many people would be happy to have two female horses, and that is especially appropriate for someone who is new to keeping. The horses are as likely to interact w/ one another as they would be to interact w/ a new male, and you don't have to worry about troublesome pouch issues. Plus, its not as though kuda fry are easy to raise. If a 29 gallon is all that is going to fit in your tank location, then get the tank and give up the idea of multiple additional fish. If you have multiple months of success w/ these two horses in the larger tank, then you can think about the addition of a single nano goby as a tank mate, but then you really are going to be at your limit.
As far as whether people really set up their systems based on the org recommendations: I bought a 56 column tank for 4 horses of a species that grows significantly smaller than the one that you have -- H. fuscus.
