Trying to come up with an answer to your post... How should I phrase this. Let me try it this way:
I have some good news and some bad news for you.
Good news first: After having spent developing with JOML for the last year I think I know my around it and could probably help you.
Now the bad news: After reading your post there are so many question marks popping up in my brain that I actually might not be able to help.
Which "number of methods that expect a Vector3f" do you mean? Methods taking Vector3f as parameters? Where are those methods? And what do their JavaDocs say they should be good for?
You seem to only relate Quaternionf to Vector3f syntactically/Java-typewise but not semantically.
And why are those methods compile errors? You mean method invocations result in compile-time errors when you want to apply some "method that expects a Vector3f" to an argument of type Quaternionf? Sure, that is just how Java types work. Vector3f != Quaternionf.
There is a demo
showcasing that camera class. You should have a look at it.