You are correct some species of seahorses can tolerate temps up to 78'. The problem arises with prolonged exposure to higher temps which enhances pathogen growth. We see it every year in our emergency forum.

As for the chiller, most of the people I know run cooling fans over the top of the tanks or over the sump during the warmer months. Just be sure to have extra buffered top off water handy. Keeping the lights off during the day will also help keep the tanks cooler. With 11 seahorse tanks, I can't afford a chiller for each tank, so I use the low tech solution. A good cooling can keep the tank up to 4' cooler.

Then again, you could do the whole house chiller and set you AC at 68'. LOL

MH & PC lights generate a lot of heat so it is better to T5's or older models of fluorescent lighting.