Composing at the piano certainly has a long, illustrious history. All of us can picture classical composers sitting at their keyboards, working out their scores for as many instruments as they wanted. Only later would they hear musicians playing the separate lines on their various instruments.
With MIDI, this is still a viable way to compose, and with a long list of advantages over sitting down to compose on a physical piano.
I think that MIDI musician/composers sometimes overwhelm themselves trying to develop music by working with all of the orchestral instruments from the very start, especially people new to computer recording. People of experience can work perfectly well that way, but I want to pass on a few suggestions for people who might like to try composing first with just one virtual instrument - the piano.
I'm talking about using DAW software, since that's my point of reference. As usual, the working methods and challenges are different for notation users.
I often restrict myself to using only the GPO piano as I write. I keep working until I'm satisfied that I've developed a solid piece of music that works with just that one instrument. At least the foundation is in place - the chords, melodies, harmonies, rhythms. I know I'll later be expanding things beyond this basic version, but it's gratifying to know that what I've written stands on its own in this simplest form.
It's not as if I've necessarily written something that could be actually played on the piano. Sometimes I keep it within that restriction, but other times I have multiple parts happening in this foundation version which would call for a talented Octopus to play.
Since we're working with MIDI, there's no reason that this piano sketch of a piece has to be on a single MIDI track. As I start developing counterparts and harmonies, I may record those on tracks of their own. I may record a bass part on its own track, counterparts on their own tracks, and so forth.
Once I'm happy that the thing hangs together, then there's the fascinating second part of the project. I start expanding the music, assigning various parts of my MIDI tracks to appropriate instruments. I can easily experiment - try some of it with one instrument, audition it with another etc.
In some cases, one of my piano MIDI tracks may be ready for trying as-is with a new instrument. In the case of other tracks that are more dense with notes, I'll select the notes I want in the Piano Roll View, and copy them into a new instrument's track.
And from there, I do passes over the tracks as they accumulate, recording volume data, vibrato, whatever else I want.
Eventually, what was at first a piano piece, is now exploded into one for any sized band/orchestra I want.
A lot of work? Sure. But it can be a very interesting way to work, at least worth a few experiments if you haven't used MIDI this way before.