Hello,
can I set a notehead style before I input notes, or can I only change noteheads after the notes are already in the score?
Depending on what you want to do, you have some control over this in Engraving Options/Notes. If you are using a Percussion staff then the options will be under Setup, click the instrument then select Edit Percussion Kit. AFAIK, there’s not a way to apply a custom notehead style before entering the notes for other styles not contained in Engraving Options/Notes.