YES!!!
I was about to post this as a new gen q and the awesome post-it told me a discussion had already begun! 
Hi! I'm an admin of a Slack community of ~5300 users.
We would like to migrate to Mattermost, but would like to do so slowly over time in a way that users still on slack can communicate with those who have migrated to mm and new users created in mm after the migration has begun and vice versa.
Ideally this communication would happen with the same latency as users are used to in Slack or close enough that real time chat on this scale remains functional.
After a critical mass of users complete the migration we would cut off the real time sync integration or even better we could let it continue until few or no users remained in Slack.
I've begun research on this and have found encouraging resources, but nothing explaining if this is possible and/or how to do it. I can share some things I've found already. FYI I work in IT, but am not a developer. I am ready to put it the long hours to try and develop this if pointed in the right direction.
Thank you and I hope you can help!
-josh