If you want to specialize, you need to be really special. What do I mean by that? You need to be the best of the best at your specialty to earn a good living. Having installed generators in the past is far from the prerequisite to be a generator specialist, unless you plan to hire diesel mechanics, go to factory schools, invest in software products, and maybe even hire a PE, etc.
I find it more useful to be somewhat of a generalist, specializing in doing work for people with money.
I like to do a tiny bit of everything, but nothing really involved in any particular niche area. If I have any specialty, it's mostly by default and no fault or planning of my own. I tend to do an unusual amount of big multi-tenant services and old apartment building rewiring. I'm really not sure I can tell you why that is, but I suspect that it's simply because not a lot of people like to do this type of work.