I was recently thinking of updating Lore Dialogue 300 Updated with the feature of "learning" topics like in Morrowind, so that not all topics are available right from the start.
The way of doing it, would be to add the "AddTopic XXXXXX" line in the result script of many Vanilla dialogue lines.

If another mod changes something in a vanilla dialogue result script, would the be incompatible? (Overwriting the other's script and vice versa?)

If it causes too much problems, I think I'll stick with adding the "Add Topic" to my custom dialogue already included.

It would create conflicts with any mods that change any of the vanilla dialogues. But, as the number of these mods is rather small and most know not to touch these dialogues without good reason, you should be able to work around or incorporate any significant conflicts. It may mean that you have a line of dialogue which doesn't give the player a related topic due to how that line is needed for something else, but this is a small price to pay. Most of the lore stuff can be learned from books anyway.