Running Sierra and Leopard on MacBook Air I just bought a new MacBook Air and want to know if I can partition the drive and add Leopard (10.5.8) so I can run my older music software.

The issue for me both the cost and I like the older versions interface better.  I still have an old PowerPC G5 which would service the Leopard software, but no current printer will support the older G5, hence my thought I could use older software, make doc PDF, then print with new OS.

Is this realistic?
If the machine is newer than Leopard, then it's not possible.

No Mac can run an OS older than itself - the drivers for it simply weren't invented.

Your only option would be to set up a VM, using Parallels or similar; but you'd have to use Lion - the oldest user version of OS X licensed for virtualisation.
Snow Leopard Server was the first ever version you can virtualise.