Hi,
That's interesting that you want to combine these two areas. I would say that sociology would certainly be applicable to some subfields of I/O Psychology. The one that comes to mind is Groups & Teams and Cross-Cultural Training. Because this field deals with selecting, training and retaining talent, it can certainly make room for research that integrates sociology with itself.
I don't really have any experience consulting for big clients, but I would refer you to the following book: Hedge & Borman 2008
http://www.masspsy.com/book/0810_ne_book_jim_IO_consult.htmlI do not think that a Master's degree would limit you to HR work, I/O is not HR, we deal with the application of psychological principles at the workplace, we do not deal with the resource aspect, we are more instrument developers. We use information from the organization (its people and environments within and across nations) to do so. You would be working with clients on different projects such as survey construction and administration; conducting training modules; making proposals etc.
I am still at school pursuing a PhD in this field. I have completed several projects on campus and a few off campus mostly in the areas I mentioned above. A beginners book I would recommend would be Levy, 2010
http://www.worthpublishers.com/newcatalog.aspx?disc=Psychology&isbn=1429223707As far as advice, I would say that you need to be intuitive and creative to be in this field. Intuitive about human nature.
Good Luck and you can PM me or reply back with more questions or further clarifications!
Nupur