If you are going to put the seahorses in the 75 as a temporary measure, what will you do with the foxface as it's not compatible with keeping seahorses?
Also, will you be able to keep the temp of the 75 in the recommended 68° to 74°F range?
It would be a MUCH better idea to refrain from obtaining the seahorses until the seahorse tank is ready. That will give an added advantage in that there would be no exposure to pathogens introduced to them from the foxface and perhaps other fish that had previously been in the tank. Seahorses often succumb to exposure to pathogens they haven't grown up with.
Are these kuda/erectus guaranteed to be true captive bred? Where would you be buying them from?