I was the other guy ogden (above) attempted knife's edge with and will be doing a third winter trip to Katahdin this February. My two cents with it would be really do not underestimate the weather because it's pretty rugged up there. I wouldn't attempt it unless you are with a partner you were comfortable with - it was good that we were both in agreement pretty quickly that we needed to turn around as we both actually got knocked off of our feet by the wind, which I had never experienced before. It's strange because at Chimney Pond it doesn't seem like it would be that windy, but once we got up on the ridge it was pretty incredible. It was very windy on the tablelands once we descended and then went up the Saddle but the wind was not nearly as bad as on Knife's Edge.
I would very generally say that no particular skills are required to get to Chimney Pond via Roaring Brook - that section just requires a willingness to suffer. The hike up and down Saddle probably just requires a little bit of comfort level in snow and icy conditions - I know I found the descent a little scary on the first 50 feet or so because if you slipped you would slide for a very long time on some icy snow. I wouldn't recommend trying Knife's Edge without at least a fair level of winter climbing/hiking experience, just because if you make a mistake you really could pay a very steep price.
In terms of what the rangers will and won't let you do, I got the sense both years they'd pretty much let you do whatever you want, though they did in a very conversational manner ask us about our experience, equipment and what we planned on doing. They are very nice and have a ton of stories about people suffering gruesome injuries and close calls in avalanches, which certainly functioned as a deterrent for me. But the name of the game really seems to be "you make the call."
In terms of skiing, the Saddle is definitely skiable and I plan on skiing it this year, but only after getting a close up look at it from climbing up and down it last year in the winter. It's a wind lip (or was last year) that drops off a bit and has a very steep pitch for the first 100 feet or so, then still pretty steep for another 500 feet or so, then mellows out. It's a lot like a much less wide open Iron Mask at Vail, though instead of soft powder it's fast east coast icy hardpack. The snow is very hard and fast, though, so I really would not recommend skiing it unless you are very confident in your abilities and have skied on such terrain before, like Tuckerman's when it's frozen after the sun passes and suddenly it's scary fast bulletproof. You could be down it on almost no time, on skis, though, versus climbing. Of course conditions could be different at different times, too.
Another far more mellow ski option is a stream bed you can hike up to from Chimney Pond and ski down. It's a fun little run, though not fast or scary. The ranger did tell us someone hurt himself very badly on that run, though, by slicing his throat open in a freak accident where he hit the edge of his ski with his face and neck. That gets me back to the main theme that runs through a trip to Katahdin in the winter, which is you really are on your own and far from anything, so one small bad decision, like pushing it on Knife's Edge, could result in something very bad happening.
Katahdin in winter is stunning, though, and one of the most beautiful places I've ever been. I plan on doing it every year for many, many years.