Here’s the deal:
The MIMC stands for MI for MC inputs. The output of the Soundsmith won’t cause any issues with the Rega. But the loading will. Soundsmith wants at least 470 ohms at the phono stage. The Rega can be set to a max of 400. Some Soundsmith users say they sound even better loaded at 1000 (or 1k same thing) ohms. Because the Rega can’t be set to the minimum loading, you’d get rolled off high frequencies pairing the Soundsmith to the Rega. The same would apply to low output Hana cartridges as well, per Hana’s specs. Lots of other cartridges will work great with the MC inputs on the Rega, just not those two.
Looking at the MM inputs, Rega specify a sensitivity of 1.7mV input for a 200 mV output. This means a cartridge with 1.7 mV input will allow your amplifier to reach rated power if it’s sensitivity on its inputs are 200 mV or less. That’s really really sensitive, BTW. Not many amps will do this. My amp has an input sensitivity of 360 mV which is pretty sensitive. A cartridge with 1.7 mV output would not allow my amp to reach full output when used with the Rega.
Another thing is sound quality. The Hana MH (2 mV output) sounds better at 46 dB gain as opposed to 40 dB gain in my system, with my phono stage.
If I were in your shoes, I’d check the specs regarding input sensitivity of your amp if you end up going with a higher output cartridge. Or, I know for a fact that the Audio Technica MC cartridges will pair perfectly with the Rega and your Pro-ject. Look towards a model with the Microline stylus. They are easier to set up than Shibata and at least to me offer the best balance of detail and surface noise rejection. Microline styli also last the longest, per A/T.