Carlo looked good in the few times he appeared last season. Just as reliable as any Leafs rearguard... that doesn't sound like much, but he is a developing prospect. I don't see it as a detriment for the team, or his skills, to call him up for a prolonged term... or the whole season altogether.
Arvedson... not such a good idea. Other than the fact that I very much dislike Ottawa, he's never been a consistent scorer in the playoffs. And at this point I think that's what the Leafs are looking for... consistency.
The Zhitnik trade doesn't sound that great. In Buffalo he was a selfish prick, and I don't see a change in venue altering his personality. The point of the trade would be to improve team dynamic and chemistry, but it would cause more trouble than its worth.