> to just copy all of Ubuntu's (or Debian's) binary packages and build a new derivative distribution on top of it
This is okay. However, copying Ubuntu’s and Debian’s packages into the same repository and then mixing them willy-nilly is not.
You can either:
a) Base your derivative on binaries from Debian xor Ubuntu, then add source packages compiled with these binaries as you like.
or
b) Base your derivative on sources from Debian and/or Ubuntu, then compile everything together with your custom packages to make sure that all ABIs match.
This is okay. However, copying Ubuntu’s and Debian’s packages into the same repository and then mixing them willy-nilly is not.
You can either:
a) Base your derivative on binaries from Debian xor Ubuntu, then add source packages compiled with these binaries as you like.
or
b) Base your derivative on sources from Debian and/or Ubuntu, then compile everything together with your custom packages to make sure that all ABIs match.