Hi sefyu - you should be fine to install XP after 7 - just like in the tutorial.
Making a backup image is always a good idea.
Make backup images of your o/s partitions on a regular basis is something you will grateful for one day.
The most efficient program for backup imaging is Macrium Reflect.
They do a free version which may be all you need: Macrium Reflect FREE Edition - Information and download
I do have a tip:
After installing XP -you may find it easier to run startup repair from 7 dvd 3 times.
Then boot into 7. Install Easybcd on 7 and add an XP entry .