Well, No species of Heterodontus is reef compatible.
Both Horn sharks & Port Jacksons are known to feed on urchins, & anomones. Also they require more turning room than sharks like coral cats. So even if they could live in a reef setting they would be very hard on the reefs inverts.
I agree the a single Horn Shark (Heterodontus francisci) as an adult, needs a tank/pond that's at least 450 gallons. Yes it's possible to keep horn pups in smaller tanks - but eventually it will out grow any tank that isn't at least 3.5-4' wide.
As water temps - Horns sharks are actually quite tolerate of water temps having been found in waters ranging from near 60 F to low 80's F. But they tend to do best in water temps that range from 64 -75 F(18-24 C). So a chiller is only needed if you use a heater, or to keep your water temp from ever rising above 80F. Which could also be done with a couple of good fans.
Also of note: Horns as well as other most Heterodontids are very hardy species, and can live more than 20 years. And Heterodontids have been around since the mid Jurassic Period (about 160-175 myo).