Which aliens would you tap if given the chance? Or for that matter, which pony tribe/non-pony race would you lay with?

I would never touch asari. They are not beautiful, they are stuck-up arrogant bitches who deserved their home world being destroyed like it was. Nor batarians, for many obvious reasons, mainly we have not seen a fembatarian.
Quarian, hell yeah. Tali specifically. Female krogan scare me.
Any pony tribe, but I have a thing for bat ponies. Changelings as well, but not the monster versions too many show them as.

Totally with you on the Quarians!

As far as Equestrian, I've always felt that a HiE would get along better with Griffins than with ponies.