I think Crocodile is one of those slaves that was part of the hunt in God's Valley. I believe he was a slave and was saved by Ivankov and Kuma, thats why he is so annoyed with them. I also believe thats why he knows so much about the ancient weapons, overheard some CD talking about it in his time as a slave.
Croco boy is a slaveboy.
i don't think so cuz if Crocodile was a slave. His body will get a slave tattoo just like Hancock. And if he does indeed have that, He will never got prisoned on Impel Down, Marines gonna send him to his CD owner and become a slave again. The fact that he got prisoned to impel down means he wasn't a slave.
Only Ivankov know his "secret". The fact that he had a "secret" with a guy that can change your gender is already sus and kinda predictable at that time. Plus he really mad when Iva gonna spoil it to Luffy and co. So its more "Personal". Hence why many fans speculate that Croc is a woman.
another point is if he know so much about ancient weapon then why bother going to Alabasta, become "saviour" of that country, plotting a war against them just to read a poneglyph about Pluton and its location. It ain't make any sense.