0.3mV and 0.5mV are close enough that there will be just a tiny movement of the volume control between them, so go ahead and set the phono stage to the 0.3mV input. The loading of 100Ω will do fine, but if you really want to experiment when you have got used to the sound of 100Ω load, you can always slip a resistor in series with the cable to play around with it. You probably know that resistive loading of MC cartridges is used to damp a peak in their response in high frequencies. The higher the resistive load, the less this peak is damped. So if you think you aren't getting enough top end, you might try a higher load, but to be honest, you'd need a better ear than mine to tell the difference.
Somebody will pop up to insist you must use a transformer and the MM input. I would save such experiments for much later in your career!