To answer the first question of Slack vs Discord: I couldn't figure out how to join the Slack channel, I think people would have to be invited (so not anyone can join). Discord is a lot more useful for this, as you can just click on a link to join.
Then, real time vs forum: while the forum is great for 'general' discussions like interesting replays or how to use starter packs, I think real time chats are better suited for questions and in-depth discussions about techniques and theory. And in a real-time setting it's much easier for people to jump in and join a convo while it is ongoing.
In any case I don't see the harm in trying - if it turns out the discord is barren or people prefer the forums, then it can be easily abandoned. 'fragmenting' the conversations isn't an issue if one of the two options is simply a better environment for certain conversations