If the little black one seems to waddle when it walks, seems to 'lean forward' more than stand upright, almost looks like something other than a chicken because of the way it stands (one of my daughters says it looks like a hallmark painting of a little black bird)
and - if its wings seem to sweep right 'round to its bum when they get developing, and its tailfeathers mostly point upright:
Yep! Its a Japanese.
Yours looks just like mine did a weeks ago. We've discovered that we have two of them - the white one developed a little more slowly and its legs are a bit longer so it doesn't lean quite as dramatically as the black one, but up goes its tail and round to the tail go its wings.
They are also incredibly LOUD when they start "complaining", for being such tiny little boogers.
And your last chick does look like our Australorp pullets did.
Was at our TSC on Saturday for hardware cloth. The only thing that saved me was that the Banty Bin was EMPTY.
Otherwise.....I'd have had to help empty it! Again.