As an historian, Polybios professed to be a champion of factual integrity (cf Histories 7.13) much in the vein of Thucydides. It is clear that he consults a variety of extant sources when writing -- written accounts, public records, interviews of eye witnesses, etc -- and personally vets those sources for reliability. Unfortunately, he rarely names those sources. I can think of one exception, from 2.56

Phylarchus [a previous written source he consulted] is by some received as trustworthy, so it will be useful or rather necessary for me, as I have chosen to rely on Aratus' narrative, not to leave the question of their relative credibility undiscussed, so that truth and falsehood in their writings may no longer be of equal authority. In general Phylarchus through his whole work makes many random and careless statements… [Particularly,] in his eagerness to arouse the pity and attention of his readers he treats us to a picture of clinging women with their hair dishevelled and their breasts bare, or again of crowds of children and aged grandparents weeping and lamenting as they are led away to slavery. This sort of thing he keeps up throughout his history, always trying to bring horrors vividly before our eyes… A historical author should not try to thrill his readers by such exaggerated pictures, nor should he, like a tragic poet, try to imagine the probable utterances of his characters, but simply record what really happened and what really was said, however commonplace… It is the task of the historian to instruct and convince for all time serious students by the truth of the facts and the speeches he narrates, the purpose being to confer benefit on learners .

Now, concerning the meeting between Hannibal and Scipio: the simple answer is that we can't be sure. Both Polybios and Livy give versions of this meeting. This guy has a nice little write-up about it.