Was Crocodile stronger at Marineford? Or was he holding back in Alabasta?

 During the Alabasta arc, Crocodile displayed a level of power that was initially considered overwhelming by the Straw Hat Pirates. He possessed the Logia-type Devil Fruit called the Suna Suna no Mi (Sand-Sand Fruit), which granted him the ability to control and transform into sand. He had a reputation as a Shichibukai and controlled the desert kingdom of Alabasta from the shadows. His strength was showcased through his battles with Luffy and others. At Marineford, Crocodile was present as part of the war that took place at Marine Headquarters. While he did participate in the battle, he didn't display the same level of dominance as some other powerful characters present. This has led fans to speculate that he might not have been as strong as initially portrayed in Alabasta. It's important to note that power scaling and character abilities can be subject to interpretation and development by the author. Oda often keeps details deliberately open-ended to keep the story intriguing.

Did Gandalf ever doubted his decision of taking Bilbo with him?

 It is not explicitly stated in J.R.R. Tolkien's "The Hobbit" that Gandalf ever doubted his decision to take Bilbo Baggins with him on his quest to reclaim the Lonely Mountain from the dragon Smaug. However, Gandalf does express some concerns about Bilbo's ability to handle the challenges that lay ahead, and he is initially skeptical of Bilbo's suitability for the role of burglar in the quest.

Gandalf ultimately chooses Bilbo for the quest because he sees something special in the hobbit, and he believes that Bilbo has the potential to rise to the occasion and become a hero. Throughout the course of the journey, Gandalf acts as a mentor and guide for Bilbo, helping him to overcome his fears and doubts and grow in confidence and strength.

In the end, Bilbo proves to be a valuable member of the company, and his bravery and resourcefulness help the group to succeed in their mission. Gandalf's faith in Bilbo is ultimately vindicated, and the hobbit becomes a respected member of the community and a hero in his own right.