With wee-slack, your Weechat installation can connect you to multiple Slacks at the same time. I use Weechat to connect to 3 different Slacks, and Glowing Bear to connect to the Weechat when I'm afraid from the commandline or on my phone. Works really well.
Glowing Bear has a preference to hide channels that don't have any activity.
As for using wee-slack in Weechat, go.py helps. I bind it to meta-g, it's a script that lets you jump to channels by typing the name. Also, /part any channels you're not interested in.
In Glowing Bear, meta-g jumps to the search bar that filters buffers as you type. Now that I think about it, a fuzzy search with ranking would be a nice improvement to reduce the number of characters needed.
