I believe any other Shichibukai that the Strat Hats haven't encountered would be able to use Hakki. What fun would there be in facing enemies that are easy to defeat? Since Sentoumaru would be the obvious first opponent they will encounter, I think it is likely that they would be able to dispatch him and his bunch of pacifistas easily. Which leaves us to believe that the others are likely much more powerful. Sentoumarou, I believe, is also proficient in hakki.