so look, it's true Kakashi said Kakuzu should have the innate affinity to use an elemental tech at such a high level. his confusion comes from this: most people have the affinity for one or maybe two natures, but they can still train to use a second or third nature. No one--except so6p or rinnegan user--can use all 5 elements so it was weird to have all 5 elements and that they were so strong. Jiraiya was NOT surprised that Nagato could use all 5 natures, cuz he expected that. he was only surprised that he wan't using them but was instead only using summonings.
So, people are born with one affinity, those with Kekkei Genkai are born with 2 and you can use the affinity you're born with in a very strong way. But, through training, one can develop tech's using another nature, but thsose techs are weaker, and no one can use all 5 nature techs. Kakashi is the one who said that through training some people can use several elements, but they are weaker and no one can use all 5. So yeah, Kakashi--and no one else for that matter (excluding Rinnegan users)--can use all 5 changes in nature. I hope this helps clear some things up a little