Thanks for your reply Steve.
Hairless MIDI doesn't seem to use much processor and memory resources so I find this way more simple, at least for now. But I don't know how I will manage that with several Arduinos...
About the solenoids part, you're exactly right. I want them to switch on and off as long as the note is on. 50ms was a value I put for my test. I figured out it was the fastest I needed (and this speed seems to be supported by my tiny solenoids). But if I could adjust their "hitting speed" with the note velocity it would be perfect. I don't know if it's feasible with switch/case though...
You must be right about callbacks issues. And that's where I was struggling. The problem with the "short notes" is that you can't have notes this short in MIDI afaik.
No problem for the stepper motors. I'm about to make them work I think (hope?).
Cyril_