Most interesting to me is his kinda Luffy-like reaction to Kaido suggesting they force Harald to do something.

Rocks did win all of his crew members through Davy Back fights. Everyone on his crew is there because of the pirate code and shared interests. I think Rocks has a red line on forcing people against their will to join him because he’s a worshipper of Davy Jones and would rather win Harald’s heart rather than attack him (which he possibly might not win, Rocks pirates are strong but I don’t think they’re go to war with Elbaf strong).

TL;DR I think pirates and marines to Rocks are fair game but people outside of that aren’t forced to join him.