Installing Mavericks on brand new Macbook with Yosemite? I want to buy a new MacBook Pro. However, it simply MUST be able to have a different operating system to Yosemite installed, as I need it to work with some hardware which is no longer supported with Yosemite.

Would it be possible to downgrade a new Macbook?

Thanks
Simple answer, I'm afraid - No.

Unless you buy a machine right on the cusp of a new OS release, it will never be able to boot from an OS that was released before that machine existed.
There are no hardware drivers for it, at the most basic level and it's a pretty dangerous thing to do according to Apple.