Hello Vincent,
Just my 2 cents,
To me, higher bit depth like 24 bits and higher sampling rates like 48Khz are really good for audio processing or impulse responses as it gives you a bit of headroom. But for instrument samples you can usually "get away" with good old 16 bits 44100Khz. Considering the amount of processing they will receive, it won't make much difference.
If you are concerned about file size and want to reduce instrument size without reducing audio quality use FLAC, I do this exclusively and it works great!
However, stereo samples make a huge difference, so I’d say use stereo as much as you can.
I do this even for instruments seemingly mono in nature (like electric basses and guitars) as they are often amplified and heard through both our ears. If you don’t do this in the samples, eventually you will have to deal with stereo imaging later with mixing and effects...