New MyCloud - mix and match?

Just bought a 3TB MyCloud.  

In this household the main “working” devices are a desktop Mac (his) and an XP laptop (mine) which is about to be replaced with a Windows 8 laptop.  The XP laptop will then join a Vista laptop and two iPads as sort of subsidiary devices, IYKWIM.  The Mac has a 1TB drive, containing a lot of music.  The Windows 8 laptop also has a 1TB drive, which is nowhere near going to get full, that’s just what it came with.

I’m unsure of the best way of using the MyCloud, but having seen the HowTo on using it as a backup, that seems an eminently sensible thing to do.  Will it work as a backup for both the Mac and the Windows 8 laptop?

It had occurred to me to use it as the means of transferring my data from the old (XP) laptop with it’s 60GB drive onto the new one… does that make sense?  Or will it somehow conflict with using it for backup?  Should I copy everything I want to transfer from the old laptop to the MyCloud, copy it from the MyCloud to the new laptop, then delete it off the MyCloud, and finally set up the backup thingummywhatsit?

And finally - is setting up a personal cloud so we can access files from the outside world a separate step we can do at a later date?