Shine is a co-founder of Soulis, it care of user interfaces and is the developer of the Android application for Souliss. I've asked him to join the discussion 
On my side it will not require a regulator at all, the inlet power will be 12V or 24V that is basically yet stable, so it could power directly the relays' coil via the transitor. So, you will have a direct input at 12 or 24V connected on the transistor collector.
I'm really happy that you decided to move on higher voltages, because a long line with 5V supply may result in excessive drop of voltage over the line.
The DC/DC require a filtering circuit, that why it use bigger capacitor. The MC33063ADR is used by Olimex in many of its open hardware boards, you can look at to find out some ideas on spare arrangement and circuits.
If you found 12 and 24V relays with the same footprint and supply the relays' coil directly from the inlet supply, you shall just change the relay to get a 12 or 24V board.
Thanks.
Regards,
Dario.