Regrettably due to the licensing stupidity there's actually two bash on macOS: /bin/bash and $HOMEBREW_PREFIX/bin/bash they do differ (as is expected by a two major version spread)

I'm firmly in the never-zsh camp, but I'd guess so long as one doesn't do something silly like use #!/bin/zsh in shared scripts you can continue to use whatever interactive shell you like

I had a college who was hard core into fish but had to be completely self sufficient because he was the only user in the office. I presume it's the same as being on Linux in a Windows shop or being on Windows in a macOS shop: no one has the day to day experience to have the answers handy