At the time of writing (december 2018), there aren’t any up-to-date and easy to apply guides on how to switch between different MySQL version on a Mac using Homebrew . Or at least, I didn’t find any.
So I picked up a few things here and there and finally managed to connect all the pieces of the puzzle. I hope this guide can help you and the future me. If anyone knows of a better way to accomplish this, I do hope they will share their insight :)
The basic idea here is that you need to install all MySQL versions one at a time and assign each its own data directory.
I am using Homebrew 1.8.5. (brew -v)