In my opinion, depending upon interest, maximum combinations are follows,
P4 - P5, Pure Managerial techniques
P6 - P7, Pure skills for Audit and Taxation firms
P5 - P7, Managerial skills with Audit on upper hand
First of all consider what interests you the most. Secondly, if you don't have practical experience in any of these areas (option papers) look at the previous exams you sat ( F5, F6, F8 & F9) and see your performance. So you'll see 3 things in selecting the ones for you and these are:
1) Your interest
2) Your experience
3) Your performance in the basic skills level paper relating to these option papers
After considering the above criteria what I consider to be good combinations are:
P7 & P4/ P5
P6 & P4/ P5
P4 & P5
I won't recommend taking P6 & P7 as these are entirely 2 different things and you may end up getting into one. So choose 2 papers which can work well as a combination.
From my point of view is P6 and P7.