I would like to used a DC/AC Solid State Relay (SSR) with my CerboGX to control small heater in my sailboat using temp from my RUUVI sensor.
I found this relay on mouser.com that looks promising.
FSC Single-Phase Solid-State Relays - Teledyne | Mouser
Will this work with the CerboGX internal relays? Do I need a diode?
It will as long as your system is a 24V system, because the relay control voltage range is 18V - 29V.
Connect battery plus (+) to one pin of the Cerbo relay and the other pin from the Cerbo relay to 3/A1+ of the SSR.
The 4/A2- pin of the SSR will be connected to battery minus (-).
Of course, depending on the logic, you will use Cerbo’s NC/COM or NO/COM set of pairs.
Oh, and I forgot, no need for an additional diode. 
