It is still worth developing a persona in your case, although it is likely you will only have one.
Promoting out-of-body experience
Personas are used during the design process so designers constantly have a character representing the target audience in front of them. This (hopefully) ensures that the user is always kept in mind, and that designers don't make any decisions or evaluations that are based purely on their expertise of the domain (ie, the designer sin).
Personas help the designer to step outside their own mind and enter someone else's.
Even one user is enough
It goes even further than that - one of the key recommendation in writing guides is to always consider the reader - novice writers tend to write to themselves. Even if you have a single reader (like a marker of an essay you write) you should write for that person and not for yourself.
As far as personas go, you would have no synthesis (development) to do if you only have one user, but it would still pay to create a persona for that one person - so it is always in mind.
It's not all about diversity
Personas are not only there to represent various audience groups of interest. They also include focal points that should inform design, and other highlights of user research.
For instance, a persona may include Key Do's and Don't; it may include analysis of expertise (such as technical or domain); it may include scenarios and key tasks. All of these still serve the designer during the design, regardless if the persona represents an authentic group of 5 people, or a diverse group of 15000, and regardless if there's one persona, or five.
If you look at the persona below, you'll see that even if this would be the only persona for a particular system, it can still support the designer in many ways.