From the specs:
•Standard 2.1mm DC jack (just like the original) with 5V regulator to run on 7V-17V power
So 24 volts is a bad idea.
Whan you say 24v relays, are you talking about the coils or the load? You will need to connect the grounds of your Arduino and watever you are using to drive the coils. The load side can be isolated.