They actually did have Fusions in Ent. And maybe the Fusion process limits lifespan?
If Kor, Kang and Koloth are any indication, I doubt it. They all lived well into their hundreds.
And the QuchHa'
aren't fusions, as we understand it - they're still pure blooded Klingons. Just altered with human DNA. That doesn't change their parentage or anything like that.