Focus groups are a useful way of accessing perceptions that may or may not be common to a group of categorised peers regarding a phenomenon or phenomena or experience that is common to them, as determined by the researcher's agenda (and research questions). They are useful in the exploratory, generative stages of research in particular in identifying common trends, assumptions, and differences. They are not useful in getting below the surface of such matters, which is where in depth interviews come in. Focus groups are often diverse individuals who have different experiences of a common object (like a product) and their "groupness" is quite specific. A homogeneous group is more  likely to produce convergent responses due to the cultural dynamics in operation that are not present in typical "stranger" groups. Not to mention the power issues - and these seem to be the elephant in the room in your study. You could do group interviews which are different from focus groups, but a homogeneous group that generates its own focal topics is likely to be difficult to engage at a critical level and would demand preternatural moderating skills. It's not unusual for "clients" in research to want to influence methods, but this seems like a power play. What are their real underlying concerns, rather than their rhetorical gloss? Is it really just time allocation?  A couple of links might help you find references or an attractive form of words: 

http://sru.soc.surrey.ac.uk/SRU19.html

https://blog.mozilla.org/ux/2012/08/when-to-use-a-focus-group-and-when-not-to/

My own view is that focus groups and individual interviews tend to get at similar content, but that focus groups give you a view of the consensus and diversity among the  participants, while individual interviews give you more depth and detail on each person. Either way, I don't see any reason why someone would suggest that you have to add a focus group to individual interviews, or vice versa. 

As for working with "similar" participants, that kind of broad homogeneity is quite common in focus groups, since it gives the participants "common ground" that they can use for their discussion. If the participants have indeed experienced changes in their work priorities, then that should give them something to talk about.

Can you say more about what you mean by working with "regulators" -- are these faculty members?

Dear Stephen,

Thank you for your thoughts and shares on the differences, especially for focus group discussions.

Dear David,

Thank you, I appreciate your feedback on this.

The regulators I am working with are those who regulate the utility sector. It is interesting to notice from the beginning that there is a power imbalance especially to the interviewer (i.e. as the interviewer has to only ask certain agreed questions, and how the research technique needs to be conducted). In this case, the interviewer will have a greater control over the respondent if interviews were held. Perhaps, building up a good rapport with those regulators will help in gaining some trust for further investigation (e.g. interviewing).

Dear Maryam,

I very much thank you for your feedback. It is really very helpful.

Hi Hafizah

I completely agree with the other responses you have received - some excellent suggestions for you there.  A few of my thoughts for you...

It is an interesting dilemma indeed both from an exploratory and ethical perspective.  It is worth reflecting here on whether you are driving your own research, or whether your participants (regulators) are doing this through selecting which questions they wish to address?

For sure, if participants in the focus group know each other, and are a homogeneous sample, then they could collaborate to only focus on areas that are of significance to them, and not necessarily help you explore areas that you have intended to focus on.  In one-one interview situations, you certainly gain more depth of exploration as others have explained more, but you are potentially more likely to be driving the research questions forward, depending on whether you take a more structured, or semi-structured approach.  However, focus groups are interesting because of the interpersonal interactions at work between participants, and this might be useful as you may be able to see how each participant in the focus group responds to others' comments regarding the change in their work priorities.  This might be why participants (regulators) wanted you to utilise this approach more than one-one interviews?

Ultimately, it is a balancing act as there are advantages and disadvantages for either one-one interviews or focus groups.  In any qualitative methodology there are interpersonal issues to consider and therefore research will likely lack a degree of "objectivity" that is generally possible in quantitative methodologies (providing questionnaires and the like).  As your research is "exploratory", a qualitative methodology is a good way forward, but if your participants want an objective focus, that in itself conceptually is worth exploring further.  What do your participants understand by an "objective focus"?  How would this be different from what they think about the topic area?

in qualitative methods, it is important to be able to work with your participants in exploring the topic area in detail, but you as the researcher are still an important part of that process.  If there is a degree of power imbalance, that will likely impact on the findings, and you might not get to explore the topic areas you intended to.  Think about a journalist interviewing a politician, for example.  If the politician is only going to take certain questions on certain topic areas, then this not only works against what the journalist wanted to explore, but begs the question what has the politician got to hide?

The difference between Focus Groups and In-depth interviews may be argued as the difference between breadth and depth. 

If the participants have all experienced "change of their work priorities" then that will give them a lot to talk about in the focus groups. Typically, the participants like to share and compare their experiences, which often provides insight into exactly the things that interest the researcher.

The main advantage of focus groups is, in one word, "interaction". You can get the respondents to state their agreement or disagreement with the responses of others. You can elicit that dynamic of your participants genuinely debating with one another about different sides of an issue. They require you to be a skillful facilitator that does not let a few loud-mouthed individuals to dominate the conversation, but draws out the quiet ones too to make sure you obtain balanced viewpoints from all. The advantage of interviews is, in one word, "depth". You can delve as deep as you want into the topic under study, but you lose the real-time interplay between alternating views that focus groups enable. If you trust your skill as a facilitator, go ahead and do the focus groups. No matter how regulated the main questions are, by asking probes and follow-up questions you can still elicit from the group what you need, more than what your "regulators" could have foreseen.

https://www.imh.liu.se/forskarutbildningar/sci-meth/course-material/1.592923/COREQ.pdf

its really useful information

I real appreciate the comments fro others but what I understand is that Focus Group Discussion entails to get information from the focus group who researcher think can deliver in depth about the pertinent topic while interview rely on the individualized information which will come from a single person.