Question: I am new to unmanaged VPS servers and in addition IPv6. Is it necessary to include 127.0.0.1 AND ::1 for localhost in /etc/hosts files for IPv6 enabled servers, or is ::1 alone enough?

Answer: 
Yes, 127.0.0.1 should be left. IPv6 is enabled, but IPv4 is still used.
On my mac, I could not access localhost. When I commented out the line "::1 localhost", I was able to access localhost again.