...
My personal opinion, if SlideWiki is opensource, then go for opensource team chat to reflect the opensource in true sense (own your chat messages then handing over to third party which might have other interests some day - better privacy). I think Rocket chat provides features that SlideWiki teams team needs.
Also see:
- https://www.blackducksoftware.com/about/news-events/releases/2015-open-source-rookies-year
- https://rocket.chat/features
...