Yes, you can install another OS on a bootable external drive and use it just for music if you'd like.

You want to go backwards to OSX 10.6? I know you cannot install an OS internally that is earlier than what came preinstalled on your computer, though it might work booting from an external drive. Probably not. You should consider moving up instead to OSX 10.8 Mountain Lion. I just checked Avid's drivers for digi002r and the last one that they tested was for 10.8 or earlier. I'm pretty sure mine was still working okay on 10.8 but I'm on 10.12 now.

Check your favorite applications to make sure they are compatible with Mountain Lion, especially LP which should be version 9.1.8. If so, you'll need to get an external USB drive and install your new OSX onto it. Then install LP9, BIAB, etc., along with the digi002r driver. After you're finished installing everything, open System Preferences in your internal OS and click on Startup Disk. You should now see your new, bootable external drive available to start up from.

UltraPAK 2021, 27" late 2012 iMac, 16 GB RAM, OSX 10.15.7
Logic Pro X 10.6.3