They're stiffer than Scratch's, MSP's, Chronics, new Mojo 90's, in fact about the only mid-fat twin tip that's stiffer is the Mojo 80, older Mojo 90, and pre-WRS Scratch BC's.
My skis before the Karmas were Atomic GS:11 race stock. Yes ghey racing skis.
I've never found the Karmas to be squirrelly, at least no more so that any other modern ski with a sidecut. Ever since the transition away from straight skis everything I've ever ridden is squirrelly at times because just like snowboards they don't like to ride flat.
I'd say at 6' you could go all the way up to 175 lbs. and be solid on the 177's. You wouldn't need to go to the next size up unless you were taller or approaching 200 lbs.