Paul...I have not (nor has my team) used Slack. I see the benefit, yet am cautious to push. Semi-related...our organization (The Salvation Army) just transitioned to MS-365 products, including slowly pushing MS-Teams. How may Asana interface with MS-Teams?
In terms of how you should use it alongside Asana. The same principles apply. i.e. decide when to use Asana (for task/project based discussion) vs. Teams (for adhoc chat and casual conversation).