SSR with low-flow switch to control heater
You only need series resistors to limit the current flow if there is a short circuit or similar.  However, sometimes power supplies require a certain minimum load in order to generate the correct output voltage.  This is somewhat uncommon, though.  In your case, the resistance of the SSR will be very high so very little current will flow when the flow switch is closed - an additional resistor would not be necessary.  On the AC side of things, you don't want any additional series resistor as this will just act like another heater, except it will just be wasting heat instead of heating the water.  12 volts across 36 k ohms would be about 0.3 mA, yes.  It's 1 mA per volt per k ohm, so 1/3 of 1 mA.