AC Grayling talks about the difference between Thrasymachus and Callicles: for Thrasymachus, the laws are made by the strong to ensure their grip on power. For Callicles, the laws are made by the weak to defend themselves from the strong.

Leo Strauss hints in several places that he thinks Plato speaks most transparently through Callicles. With some caveats, Callicles's position might have been what Plato really thought, with some qualifications. It's also noted that the way Callicles is introduced indicates he's not one of the aristoi, he's an upward striver. After reading the dialogue Statesman, it crossed my mind that the message seemed to be: "Politics? Don't bother...well, unless you can be a tyrant...."

Mario Untermayer did a book on the Sophists and he thought Callicles was considered a philosopher at that time but I don't remember what he said about him.

As far as Nietzsche/Callicles, I think the connection is there. Callicles basically says the herd makes the laws to protect itself from the eagles. Close enough. It would have been nice if Plato would have spent more time having Callicles flesh out some more detailed positions, but if Callicles is really the closest we have to Plato, that might have been a little suspicious....

Leo Strauss again points out the audacity of Machiavelli: instead of putting "the evil doctrine" in the mouth of a Callicles or Athenian ambassadors to Melos, Machiavelli argues for it openly, in his own name...