My first concern is whether Pipewire plays nicely with PulseAudio and Jack? If I install Pipewire will it break applications that expect to work with PulseAudio and/or Jack?

It seems that Jack clients will be able to connect to Pipewire: https://github.com/PipeWire/pipewire/wiki/JACK

They don't seem to know yet whether the same should be possible for PulseAudio: https://github.com/PipeWire/pipewire/wiki/PulseAudio