well the first boss is the heartless.
They are in Agrabah and Neverland (Clocktower)
Squirming Evil
I don't think you can. If you really want to redo the bosses, then you have to play a new game
Kingdom Hearts is a fairly long game. Kingdom Hearts has many things to be accomplished such as Gummi Ship routes, the various worlds found in the game and the optional bosses.
Kingdom hearts II final mix is an extended Japanese version of Kingdom hearts II with more bosses including Terra from birth by sleep and a bonus secret ending.
Its like KH2 but with more seceret bosses, keyblades, and abilities.
well there are a lot of secret bosses in kingdom hearts. most of them are in the Olympus coliseum's matches like sephiroth and the ice titan. one called Kurt zisa is in agrabah you have to fly over with the carpet and you will get attacked. and the last one is phantom who is in neverland at the clock tower so you have to fly with tinker bell. if you have the final mix you get a lot more secret bosses
The time line is: Kingdom Hearts: Birth by Sleep, Kingdom Hearts, , Kingdom Hearts: 358/2 Days, Kingdom Hearts: (RE) Chain of Memories(at the same time), Kingdom Hearts II, Kingdom Hearts III: The Keyblade Wars.
If you mean for bosses, then it's 300 hp.
you get them after beating bosses. they just give you some backstory on ansem and the heartless.
You find all your answers for Kingdom Hearts on "wiki Kingdom Hearts"
after you finish hollow bastion then you can fight sephiroth, Kurt zisa, phantom and the ice titan, they are the secret bosses