I agree with Riehl that we can, essentially, co-dominate the Sirius sector, peacefully with the humans.
Just as we can co-dominate one Human body, as Zyliath said, we can co-dominate the Sirius sector.
Every mutation starts out small - and we don't know if the Nomads have been around longer than the millions or billions of years you need for such an evolution.
Also, you're forgetting the "mechanical" part of biomechanical.
I can draw a similar comparison to the Borg Collective of Star Trek, who - like the Nomad Peacekeepers - were once a violent race seeking only to assimilate.
Now though, they have another sub-faction - the Borg Cooperative - who are still Borg, and are still a Collective, AND YET they are not.
It all started with one random act of kindness between someone who hated the Borg (namely Jean-Luc Picard, my favorite Trek character - and an off-topic tale for another time) and a disconnected Borg Drone who became known as Hugh.