Boot into Mac OS X Snow Leopard 10.6 on a 2015 MacBook Air I have an external SSD containing a bootable installation of Mac OS X Snow Leopard 10.6. It also contain some software crucial to my use.

Can I boot into it using my 2015 13" MacBook Air?
Unfortunately you can't.

OS X Yosemite 10.10.2 is the earliest OS supported on your 2015 MacBook Air. Mac OS X 10.6 Snow Leopard is too archaic (released in 2009) and you won't be able to boot into it on your 2015 MacBook Air.