> 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 D

> 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.