If Bellamy is already going to join someone, despite the fact he used to have his own crew, I doubt it's going to be Luffy. That would simply be too awkward, confusing and also humiliating for Bellamy. Even until the very end he wouldn't betray DD and consequently fought Luffy despite that he clearly said to himself that he knew he had been admiring the wrong person all along. However supporting DD to the every end to him was his own way to save his pride, that he at least wouldn't be a guy who would turn his back on his 'captain' when things take a turn for the worst. So him joining Luffy after the battle then seems unlikely as Bellamy would probably consider that as something that tarnishes his pride.
Law on the other hand is a different matter as him and Bellamy are the same in the sense that they share a complicated past with DD, so Law and Bellamy would likely be able to understand one another better than Bellamy and Luffy. Also Law has a history of picking up stray pirate captains
You really got to give it to Oda though, he succeeded in creating such depth for a character like Bellamy despite having giving him only a very limited amount of screen time.