How to install Yosemite?

I have a 2012 Mini that I think was originally shipped with OS 10.7
On that one I updated the OS to 10.11.6 (El Capitan)
Now I want to check some old floppies and found out that Apple discontinued support for those after OS 10.10

Luckily I made a Yosemite installed on a USB flash drive using diskmaker
OK I figure, no problem, I will just install Yosemite on an external, then boot from the external into Yosemite and read my floppies.

The extermal is a USB 3.0 drive with 4 partitions, each one 500 MB
I used Disk Utility to erase one of the partitions to install Yosemite on it

Plugged in my USB flash Yosemite installer
The installer icon comes up the way it should
But when I double-click on it, instead of asking me where I want to install Yosemite it comes up with this error message:
This copy of the “Install OS X” application is too old to be opened on this version of OS X.

Hmmm
So then I decide to restart holding down the option key but that doesn't help.

I'm obvious too dense to see the forest for the trees because this says I should be able to install Yosemite on an external drive with the Mac running OS 10.11
https://discussions.apple.com/thread/7470094

I'm at the step:
boot the external installer disk and install Yosemite

Is the Yosemite installer supposed to show up when I reboot the Mac while holding down the option key?

PS: The external hard drive also has a 10.11 backup clone on it.
Strangely enough neither the 10.11 backup clone nor the 10.10 USB installer show up as a start up disk in system preferences.
I remember on older versions of the OS they always used to - at least the backups.

The back up clones DO show up when I boot up holding down the option key and I can then boot the Mac on any of the back ups.
Is missing the back up drive options in the start up disk window another one of the new features of OS 10.11?

via ehMac.ca https://ift.tt/2vl3Gp6