Seems like you only have surface knowledge on these characters;
Kuro is hailed as having the second highest IQ in the east blue, behind Ben bekcman and also most analytical pirate in the east blue too. He's implied to have outsmarted the Marine on multiple occasions and made 100 of plans hence his name "Kuro of a hundred plans". Jango stated that Kuro could kill 60 skilled assassins with ease via his Shakushi, even East Blue Zoro couldn't catch a glimpse of his speed. In a SBS someone asked if Kuro's Shakushi is as fast as Soru, Oda said it is as fast as Soru, but Kuro doesn't control properly.
Krieg is considered as strongest man in the east blue (in military strength) and had a terrible reputation for his devious and cunning tactics. He has a fleet of 50 ships and over 5000 pirates, and they allow faithfully follow him with roughly over 10+ years experience as pirate. The fact they didn't considered to revolt against him, despite his awful personality should tell you alone that he definitely had the strength (military strength) and skills to make them follow him till the Grandline.
If you want I can continue just say so I deliberately left out the others. Skills doesn't necessarily mean Character x is stronger than Character Z for example, these pirates doesn't have any proper teacher to train them, they became strong on their own and most of Luffy & Zoro's foes are far skilled than you imagined. Don't get me started with either Luffy or Zoro, especially Time-skip Zoro who is personally trained by the world strongest swordsman in One piece and defeated Humandrills who are capable of imitating swordmen's techniques and skills after seeing them once, they perfectly imitated Zoro's three swords style after seeing them once. And Zoro's training consisted of fighting against them everyday.