I want to have a Living Pokédex and I have some questions.

1. What are the most common rules for a Living Pokédex?
2. What games do I need to complete one (without GTS, I want to be 100% sure that my mons are legit)? I read that you can get everything with just XY and ORAS, except for some legendaries, is that true?
3. If so, how can I get those missing legendaries?

Thanks in advance!