[Plush]

I would mic the soloist in stereo for sure. The reason to do that is that when you add the soloist in to the stereo picture, you can pan them to their position without ruining the stereo from the main pair.

Pan position of the stereo spot mics might be, for example, hard LFT. and then the other mic at 9 o'clock or 10 o'clock.

This allows for flexibility and stereo pick up of the soloist gives more dimension to their sound.