First answer: If you are able to explain the reasoning for a particular headcanon, then it is likely to be accepted. In this case, it sounds like Nanoha knowing how to create Cartridges is a reasonable headcanon, but please explain your reasoning when requesting the Echo!

Second answer: If canon off-shoots do not contradict her main source of canon (ie. the main TV series), then they can be used as supplemental material for her history/personality/Echoes.