ProTools 8 LE on El Capitan. Need solution.

I have an early 2011 Macbook Pro with upgraded RAM (upgraded from 4GB to 8GB). I recently upgraded the operating system to El Capitan 10.11.6. Now, I cannot use my older audio recording software (ProTools 8 LE) and audio interface devices (Mbox 2 Mini, Digidesign 003 Rack) for recording voice and music. I Googled until my fingers are blue and seems a common fix is to install a bootable older version of a Mac operating system, or 'downgrade' from El Capitan back to an earlier operating system. I want to create a bootable older operating system but am having no success. Apple doesn't let you go back and download just any of their old operating systems. In my case, only Mavericks came up as a free option for download. So, I did that, plus I paid $20 for Apple to mail me a disk with Snow Leopard 10.6.3 on it, since I saw somewhere online that 10.6 was the last version supported by ProTools 8 (though I think I was using my ProTools 8 with Yosemite before El Capitan).

My problem is, neither Snow Leopard or Mavericks will install. I created the proper partition on my hard drive as well as on an external 32GB thumb drive and had no luck getting the install to work on either. In both cases, I get an error message immediately when trying to install Mavericks saying: "This copy of the “Install OS X” application is too old to be opened on this version of OS X". For Snow Leopard I get an error message saying: "You can’t use this version of the application “Install Mac OS X” with this version of OS X....You have “Install Mac OS X” 23.1.1."

So, do I need some older version of whatever "Install Mac OS X" is?