Hello,
Adding the resistor is OK, the audio source will not short your varactor.
Basically you are right, they form a series circuit. But C5 is in parallel with the base bias resistors and the input of the transistor with emitter resistor/capacitance, so the effective capacitance will be less then 6 pF in this case.
You should increase C5 to get a better match to the input impedance of the amplifier. the Q factor of the LC circuit will drop to a very low value now.
I think you also must increase C4 somewhat to get more gain from the amplifier. Try this with simulation. don't make it to be a short circuit, as some capacitive effect in the emitter helps these circuits to oscillate better.
I expect this circuit to be not very stable as the temperature dependent parasitics of the transistor (BE capacitance, CB capacitance) are not small with respect to your resonating capacitors.