1 There are regulations and rules that guide the govt worker determining what is the governments. When there is a dispute,such as with Clinton, a Court will decide.
2 the Clinton tapes were made for the personal biographer hired by Clinton. They were not government recordings put in for government records is what the judge ruled. They were not marked classified, govt documents that can not be mistaken as being personal.... This argument falls flat.
3. Trump could do so...though his constitutional pledge to do no harm, would likely conflict with a mass declassification without ever telling anyone in the Agency that classified the documents.
Basically, he can declassify, but he has to, under all condition, follow the rules in place, to do so. This isn't a Willy nilly process,nor should it ever be....you know better than buying in to Trump's bull shit on this.