1P8: Rearranging custom fields?

The label is considered when filling forms, yes. Whether or not it makes a difference depends on the form and the value of the label. The field type can also be considered when filling forms, but that's the invisible part you can't currently change in the desktop app.

I'm not sure how much of a difference the field type of these text-like fields makes right now, but I could see a scenario where for email fields (like with the username field) we pop up an autocomplete box that offers other email addresses you've saved in 1Password, for example.

All that to say, I do think we should allow you to at least be able to see the type of a field so you can determine if it needs to change.