Is it truly free and open source?

Yes, both podman and podman-desktop are FOSS under Apache-2.0 license. And they prefer more open image hosts like quay.io by default (dockerhub is still available OOTB for it to be a drop-in replacement of docker)

Does it run only in Linux?

No, it runs on Linux, Windows and MacOS (through VMs on the latter two) and my experience is that it works better than docker on non-Linux machines.

The headscale integrated one is mostly enough, you can choose to include the tailscale official ones with their URL as well.

Difference is that YOU CAN BE THE ADMIN whenever you want while still being able to talk to others. Over.