I've been using the Legends page on Serebii to decide what counts as a Sub/Legend/Myth.
So Zygarde would count as a Legend since that's what it is on Serebii.
Tapus would be Subs.
As for the rest of your comment, I really can't say anything right now, the future of PokeBreeder is a little uncertain right now and I don't want to go making a bunch of plans for chapters I might never get to.