Tough to answer without more description of the setup, what you plan on doing, etc.
Generally, rougher compounds on firmer wheels (red/green on muslin or flannel/muslin, white on a softer wheel, Carnauba on your softest).
If you go softer than recommended, things will just take a little longer. So I'd err on the side of cotton/flannel/whatever. Particularly if you are talking pipe restoration.