I don't think haki is common at all and since wb "pirate fodder" never actually injured Crocodile I doubt they have it. Amazon Lily is a special case. Sure, the further you go into the New World the more likely you are to find haki users, but even then they aren't very many.
You do not know whether or not the WB fodder actually injuried Croc. But even if they didn't, it doesn't matter. What matters is that they were able to push him (and Daz Bones) back without water and apparently without seastone. It's pretty obvious even fodder members on Whitebeard's ship could use Haki.