Can you mix the two approaches provided in the 2008 Edexcel specification?

The simple answer is 'yes' - although it would be better to do this by whole assessment units rather than by topics, because tracking the content covered might be complicated.

The SNAB approach and activities encourage development of skills not necessarily formally assessed. We think this means SNAB students develop a degree of autonomy and motivation as a result of following this course.

Here is a page with a link to the Edexcel 2008 specification.
SNAB post-2008