You can use a ULA if you want to. That's essentially the IPv6 equivalent of a private IP.

Why though? Having the same IP for both internal and external solves a bunch of issues. For example, you don't need to use split horizon DNS any more (which is where a host name has a different IP on your internal network vs on the internet). You just need to ensure your firewalls are set up properly, which you should do anyways.

Never claimed it was, please quote me where I said as much