A Quaternion can represent an orientation. I struggle to understand what you mean with āa quaternion for rotation speedā because that is not an orientation and I suspect that it doesnāt actually represent what you think it does.
What you can do is interpolate between two quaternions representing orientations. Say that you have a current orientation and another quaternion representing the desired orientation, then you can do a āslerpā interpolation to find another quaternion inbetween the two.

Yeah, applying scaled euler rotation to a quaternion is not 100% straight-forward. I donāt think JME has a built-in for it.

SimMath does it in the Quatd.addScaledVectorLocal() method:

Edit: and note, I got that from a book. I donāt pretend to really understand it but I know it works because my physics engine uses it all. the. time.