Not sure if it’s possible Native, never tried to use it. If not people have done various experiments controlling the Finetune of an Instrument via MIDI, so if there’s not a Tool yet which quite does this it might only take a little hacking of an existing one…
I don’t think you can on an instrument be instrument basis. You can make all instruments monophonic by turning off Chord Mode though. I don’t know how this will make it behave if trying to play two instruments at once though (ie split or multiple keyboards.)
Hmm I am thinking it might be best to just use a sampler plugin. You wouldn’t happen to have a good recommendation for Mac? Something lightweight and free?