Let me ramble for a bit here. As I understand, you want power to the
heater available at all times when the thermostat calls for heat and
when it does you want to interrupt the power to the compressor it it
will not run until the heater goes off. So, what you need is a relay
that is operated by the thermostat. Put the coil to the relay across
the (probably) 24 vac circuit that will operate the relay contacts.
Run the power to the compressor through the relay contacts so that they
are open when the thermostat is calling for heat (furnace is on).
Simple DPDT relays are cheap and available from Newark.com as well as
Grainger or others. I'd get 15 amp contacts for the compressor power
and a relay matching the voltage of your thermostat circuit. Simple
yet elegant.