What you describe is mixed-methods research design called a Sequential Explanatory design. See a basic handbook on mixed methods for more information.
Creswell, J. W., & Clark, V. L. P. (2017). Designing and conducting mixed methods research. Sage publications.
I am not sure what you mean by "research philosophy", but the idea behind mixed-methods is that the weaknesses of one method are remediated by combining it with another method. Both methods might originate from different worldviews or research paradigms. Mixing methods, then, is almost by definition supporting a pragmatic or pluralistic view on research.
The mixed method is known as Triangulation and I refer you to use following reference.
Triangulation and Mixed Methods Research: Provocative Positions
Donna M. Mertens and Sharlene Hesse-Biber
Journal of Mixed Methods Research 6(2) 75–79 The Author(s) 2012 Reprints and permission: http://www. sagepub.com/journalsPermissions.nav DOI: 10.1177/1558689812437100 http://jmmr.sagepub.com
For I like working with quantitative methodology to make a part of the research that serves as a framework or descriptive context, and the qualitative part to deepen the subject of study. But they can also be two complementary parts of the same object of study.
The epistemology or philosophy that justifies the use of qualitative methodology is that of the social sciences: ethnography, phenomenology, hermeneutic, theories of interpretation and theories of social construction.
Each of the two components of a study of mixed methods (that is, the qualitative and quantitative components) has its own philosophy (constructivism and postpositivism) and these philosophies are not combinable. therefore, the philosophical approach used in mixed methods is pracmatism.
I don't think you've quite given enough detail of the research question, the nature of the questionnaires or of the interviews to give you a categorical answer. It sort of depends how 'mixed' the methods are.
I think you're asking about research paradigm, which really comes before methods conceptually (IF you accept the concept of paradigm in the first place - see below). That is to say that your question arises from a particular way of looking at and understanding the world and then your methodology should ideally be consistent with that worldview.
It is quite possible from what little you've shared that you are using methods that are entirely consistent with a realist/objectivist position. This is reinforced to some extent by your mention of triangulation which can be seen as an attempt to ensure objective accuracy.
However, there is an argument that the whole concept of 'paradigm' is unhelpful. This is characteristic of a pragmatist philosophical position (which, in short, prefers utility as a test of truth rather than some analytical argument about epistemology).
There are a number of useful discussions already on ResearchGate which you could view in order to understand this better. In each of the following, Prof David Morgan consistently argues the pragmatist position and he has useful papers on the topic available through his ResearchGate profile.
I agree with Peter that you have not mentioned about your research topic and questions and in absence of that what one puts somehow becomes a bit speculation. It seems that you are more inclined towards looking for objective reality , which is basically the domain of positivism. However , there are questions about the possibility of adoption of singular paradigm and many people would like to go for 'paradigmatic synthesis'.
In terms of research philosophy and since it is a mixed-method research, it depends largely on your research questions and hypothesis. For the qualitative approach, you can choose either grounded theory approach or phenomenological approach. In terms of the quantitative approach, it will depend on the kind of hypothesis and how you intend to quantitatively measure the hypothesis. For now, since the questions or hypothesis are unknown, the research philosophy has to be pragmatic than one which is based on a sound theoretical or conceptual framework or deliberation.
The mixed research is based on the two types of scientific research approach: qualitative and quantitative. When the second case occurs (related to the collection of data, which often require statistical figures), the quantitative (which refers to the description of direct observations of natural phenomena) usually occurs. See that both approaches are present. My opinion is that the quantitative always has a mixed level, which does not happen with the qualitative.
It depends on who you are interviewing and the kind of research you are undertaking. For example, if you are interviewing indigenous peoples and you are an outsider, then there are typically ethical guidelines which researchers are asked to follow, for the purposes of returning the knowledge back to them once it is published.