When in doubt, trust Vincent.
Personally, I avoid the soft "c". I like to imagine the Cimmerians as the Cymrics, even though they are based on a different people.
The internet says:
"The Cimmerians or Kimmerians (Greek: Κιμμέριοι, Kimmerioi) were ancient equestrian nomads who, according to Herodotus, originally inhabited the region north of the Caucasus and the Black Sea, in what is now Ukraine and Russia, in the 8th and 7th centuries BC."
So if the origin is Greek, it would still use the hard "k".
Just to be different, I talk about "KOH-nan" normally (as Arnie says it), but in game I use "kon-an". That's when players sit up and pay attention. He's also shorter, but no less formidable "in real life". i.e. when the players met him in the Tide of Blood.
On the topic of acents, in Tide of Blood, there is a journal of a dead explorer. I read it out in my best Steve Irwin voice. It totally ruined the tension and mood, but it went down very well. In my defence, we were waiting for all the players to turn up, then I went back to savage drop bears and killer crocs. "Crikey!"