Yes you can. But you dont really need so many (30) for interviews, 20 should be fine. More than 20 interviews could lead to information overload.

Thank you! @Sachin. I will be looking at the government public sector so I figured I use the largest civil Ministry to represent views of the supervisory staff. Would 30 still be too many for qualitative?

To add what Sachin Suknunan said, you can interview until the point of saturation (i.e. where the next participants provides no new information) for qualitative research. Refer to the following useful article: Saturation in qualitative research: exploring its conceptualization and operationalization - doi: 10.1007/s11135-017-0574-8, but there are many other sources.

Data source triangulation implies that you can compare the results for the same research question(s) from two different methods. If each of your methods can independently answer the same question, and if those results are directly comparable, then you could call that triangulation. Note, however, that it can be very difficult to meet those conditions.

Triangulation is to utilize multiple sources in an investigation (e.g., different sources of data, multiple types of data, or multiple participants) to enhance credibility accommodating a query. There are various types of triangulation; however, the key in understanding triangulation is “multiple” and/or "different" in terms of data, time, space, source, mode, and/or people. Since you are planning to use a survey and semi-structured interview to gather multiple data from your study participants at different times (and probably in different places), addressing one question, it can be considered a triangulation (data triangulation). A reader-friendly classification of triangulation was rendered by Denzin (2009).

Reference

Denzin, N. K. (2009). The research act: A theoretical introduction to sociological methods. Routledge.

Denzin has modified his position on triangulation over the years.

Qualitative research entails open-ended questions to informants to collecting and analyzing answers on non-numerical data (e.g., text, video, or audio) to establish patterns to understand concepts, opinions, or experiences. Qualitative research is applied to gather in-depth insights into a problem or generate new ideas for research.

Interesting read on triangulation 2.0

Thanks for sharing, David L Morgan

Yes, closed represents quantitative data while open-ended gives qualitative data. Therefore, a mixed-research approach necessitating triangulation.

A different approach would be to collect your survey data using close-ended questions, and then when you have the results from the survey, collect follow-up qualitative interviews to help you understand the quantitative results. In mixed methods research, this is known as a sequential explanatory design (QUANT --> qual).

There are several reasons for using this approach. First, it is easier to apply than triangulation, where you you have to compare two very different kinds of data. Second, triangulation basically amounts to answering the same research questions twice, whereas a sequential explanatory design generates new insights. Third, if your triangulation produces result that diverge rather than converge, it is not clear how to proceed.

No, it is not triangulation.