There actually is an explanation regarding crossbreeding and harming the mother. It was talked about earlier in the thread, with mentioning how they usually take after the mother primarily. This is how, for example, a Lalafell could crossbreed with a Roegadyn. The baby would still be Lalafell sized, even if it would take on some Roegadyn characteristics. That being said, it's likely that there would be a decent chance of mechanical dysfunction regarding such a pairing, but that's another issue entirely. Despite their rarity, I would love for them to be available as player options at some point, since player characters have a larger justification to be 'special'.