I don't see any reason not to consider a member of your chosen family as a good choice for legal guardianship. I'm going to wear it for this probably, but I see the choice of "God Parent" as a bit more tricky.
Here's my reasoning.
Legal guardianship can be adjusted to reflect changes in dynamics with a simple document. Poly relationships may be more prone to changes in family make up so while changing who has legal guardsianship is easy enough, I don't know if you can change the person chosen as "God Parent" so easily.
Regardless, as you said, this is long way off and when the time comes you'll be in a better place to make that decision.
Playing the Game of Life with Monopoly rules.
Monogamy might just be in my genes
Poly Events All Over