There was nothing wrong with the Notes idea, it was just the awful user interface and resource hog client app. And all that messing around with knowing the exact path of replicas. And the stupid client key thing that meant you couldn't access from more than one location. And the email integration.
What Slack does well is an easy to user interface, so matching the two sounds ideal.
We run cross-company Slack already, but it means having a second, shared account and switching in the client. Just adding external people to your conversations sounds perfect.