I think Koloss-blooded means that it is a mix between koloss and human. If it happens once theres nothing saying it cant happen again,
but we dont know anything about modern day koloss more than that they form tribes and are now a true breeding race, how they even have children with humans is a bit of a mystery. But really, it is your story, go with it.
EDIT: I looked this up and I was wrong about Koloss blooded being half human XD
Anyway Miles has a koloss blooded in his team who is a pewterarm, allomancy is a human trait, so I think the proof from the books are pretty much in your favor.