I think the editor is not, but the player is. And there are also DOS compilers that create a musica file from a textfile, so you don't even need the editor.
I think the editor IS. It supports SCREEN0 WIDTH40. Maybe not every MSX1 has a diskdrive, but that same reason can be given to PSG Tracker as well.
BiFi: And what about certain memory requirements?
What about it? In Japan all MSX computers have 64 kB (except for the MSX turbo R machines). So MusicA should work fine then. Maybe except for the MSX 1 machines having less than 64 kB...
Well then. I think we can conclude the best music program to write PSG only music would be MusiCa then. It's MML and no tracker, but still. If you desperately wants to track you should try PSG Tracker indeed...