128 first Unicode characters is compatible with ASCII.
Is 256 first Unicode characters compatible with any extended ASCII standards?
Is 512 first Unicode characters compatible with any other coding standards?
The first 128 code positions have been taken directly from ASCII. However, there are slightly different versions of the ASCII standard and, moreover, various 7-bit codes (now largely unused) have been called “national variants of ASCII”.
There is no “extended ASCII”. ASCII is a 7-bit code. Most character codes used in the world have positions 0 1o 127 taken from ASCII, so they might be called “extensions to ASCII”, but this would be rather pointless.
The next 128 characters in Unicode have been taken from ISO 8859-1.
(Regarding control characters, in C0 Controls and C1 Controls, the standards formulate things somewhat differently. In any case, the meanings of most of those control characters are defined in other standards and specifications, as they are not really a matter of character code standardization.)
There is nothing special about the next 256 characters in terms of relationship with other standards. Their allocation was not based on any previous standard.