The issue I'm having is specifically this - https://askubuntu.com/questions/671159/bootable-usb-needs-cd-rom@adamjackson If you did a Cmd + Opt + O + F, that will put you into Open Firmware, which is a CLI. Unless you entered multi-boot, how did you boot from a drive with a penguin logo? Seeing that should have required the boot picker, which is accessed with just Opt (not Opt + Cmd + O + F).
You can burn them to disc if you wish, but if you followed the Wiki's USB boot instructions, installing via USB is usually faster and pretty straightforward.
Also, the Ubuntu 16.04 download offered in Distributions is supposed to be of 16.04.6, not 16.04.5.
ubuntu's installer is asking for a CD-ROM and I was in the OF Picker as you described. It definitely recognizes the USB drive has ubuntu on it and goes through the installer loader.