Hi,
There is nothing stopping you using mixed methods – if you have a read through any good qualitative research general book you will find reference to mixed methods works and justification of such a method. Collecting a mix of data is also good for triangulation purposes.
Other examples of qualitative research which might work include focus groups, ethnography (conducting observations), participant observation, action research. Really depends on the topic being studied. Have a search for journal articles on mixed methods and qualitative research and see what has been used before.
Also your interview needn’t just be a series of questions – things you might want to explore are Personal Construct Psychology (although it is not easy to master), or using different scaling methods such as Likert scales etc.
Hope this helps. I hope you have your interviews already either done or organised. My experience has shown me that they take longer than you think to organise. Then they need transcribing!!!
E