That's pretty difficult. The Chofetz Chaim is rigorous on the topic of speech. To the point where it is difficult to say anything! Basically, if you know something absolutely factual about somebody else, and if that knowledge would affect the decisions a third party would make concerning that person, if the third party knew it, then and only then you MAY (not 'will be) be justified in saying something to the third party. Maybe.
However - I had a boss once, whom I will call X.
I like to say, I learned a great deal from X.
(and I did. One of the things I learned was why some people shoot their co-workers!)
I think the main idea is that you should consider very hard, whether the person(s) you want to tell things, actually 'need to know'. If they actually 'need' to know, then you can tell the bare minimum.
It seems to me that anything I might say has the potential to influence anothers decisions. Even if that means they think of what I've said years on and pause for any length of time before making a decision.
yes, and that's the Chofetz Chaim's point - that words are of near-infinite potential, so we ought to consider very carefully before saying anything.
OTOH, sociologists tell us that 'gossip' (as in, talking about everybody who isn't present with everyone who is present!) is the way communities exhibit cohesion, and that this kind of behavior is a control on people within the community, because reputation is so precious. When everybody knows everybody's business, it is hard for cheaters, connivers, wife-beaters and so on to get any serious evil going - because the entire community is always watching and talking about everyone.
The CC was against all sorts of 'gossip' even the benign and 'positive' kind. Telushkin talks more about how we use what we say properly - to heal and help and bring together.
That said - I have found, over the years, that you almost can't tell somebody something they don't want to/aren't ready to hear. You can say something repeatedly, and they simply don't hear what you are saying anyway. So you have to consider not only IF you should say something, but HOW you should say something - and when, and where, and why.