It Is Technically A GM Call....
But I would tend not to allow it, unless someone came up with a truly compelling justification.
An adopted ancestry means just that-- you're an honorary kobold by virtue of having been adopted into the culture. Whether that means actually being adopted at birth or as a child and raised among kobolds, or having joined a tribe years ago and been fully accepted and acclimated into it, or something similar. You are therefore allowed to take ancestry feats from any actual ancestry you have, and any ancestry feats from the adopted ancestry as long as your body will support them:
You’re fully immersed in another ancestry’s culture and traditions,
whether born into them, earned through rite of passage, or bonded
through a deep friendship or romance. Choose a common ancestry. You
can select ancestry feats from the ancestry you chose, in addition to
your character’s own ancestry, as long as the ancestry feats don’t
require any physiological feature that you lack, as determined by the
The emphasis is mine, and it's in two parts:
- Your physiology needs to support the feats, and,
- Your GM determines what your physiology supports.
Some of the Kobold Feats are pretty clear that they need kobold bodies to support them:
Kobold Breath, unfortunately, is not so clear cut. It comes down to whether or not the GM considers the Kobold Breath as something inherently dependent on kobold physiology that some kobolds are born with, or whether it's more of a spell-like effect when anyone with any body plan can learn.
The latter position, though I also find it counter-intuitive, can be supported by noting that the Kobold's Breath feat has the Arcane and Evocation tags as well as the Kobold Tag.
But the emphasized text from the Adopted Ancestry text, in my opinion, trumps all: It is, per the rules as written, explicitly a GM call.