One Piece Chapter 530

[poopdeville]

Yeah, but Ace is a captain in Whitebeard's fleet. He has his own crew to follow him. He wouldn't be as cool if he ditched his nakama.

[Augury]

I don't see anybody joining the crew from this arc. Most of the possible candidates are too powerful, don't have a defined role on board, and have their own allegiances to look after. I think Oda planned to gather all of who Luffy considered necessary crew members (navigator, cook, doctor, musician) before rolling out his "main storyline" which is this war & change in the balance of power that is going to occur. New crew members also usually interact with the rest of the crew before eventually joining, and the Strawhats are all split up now.

About Buggy: I thought he was brought down to level four for torture when he was initially caught before being imprisoned on level one in accordance with his low bounty.