I've got a ton of latest rumors that NPCs in my city should normally say, but they don't. Why? I have that function thing for each rumor set to Global -> Random100 -> some random number, but it doesn't seem to help, all they do is talk about Solstheim, and none of my rumors ever come up.

Latest Rumors work like any other dialog topic in the game. From your description it can be a number of errors that prevent your lines from being used, for example they might not be located high enough in the info list or the conditions might be incorrect. I would suggest taking a look through http://www.tamriel-rebuilt.org/files/tuts/dialoguetut.zip as it is concise, detailed, and to the point.