Hello Josh,
First thing first, maybe you know, maybe you don't. So sorry if you already know.
If you have access to a recording device which can record in 24 bits please use that and be sure to have good amplitude margin by not setting the input volume too high to prevent clipping.
Decent digital 24 bits recorders offer an absurd noise level floor so you shouldn't need to be concerned too much about the volume.
You can always boost the volume globally afterwards but digital clipping is a problem...
Yes, clipping can be somewhat fixed with software such as Isotope RX, but it's tinkering.
To me, 48 kHz sampling rate is enough.
However, record in stereo if you can, it makes a big difference.
Try as much as possible to record all notes and everything in the same conditions - don't tweak settings between takes.
I don't know about the calliope, but many instruments can have their higher notes progressively quieter and that's fine, it's natural.
Unless the instrument is really broken and the volume between the notes is so different it really becomes a problem, you shouldn't need to match the volumes of the notes.
If you aim for something realistic, you should maintain the relationship between the notes.
If you have the time to record each and every note, it's already very good.
Don't try to be too fast and don't overlap the notes! try to leave 2 seconds of complete silence between notes, it will help greatly when editing.
About note length:
It seems to me that calliope is typically an instrument where you should use sample looping.
For example, I programmed a pipe organ and note samples are between 2 and 6 seconds long including the release of the sound. The samples are all looped.
Low notes needs to be typically longer than higher notes to let the sound live and evolve properly.
The releases can be used later when programming the instrument to get a more natural feel when releasing the keys instead of just a typical fade out with ASDR envelopes.