Rockwool and mineralwool are the same thing; my understanding.
they are both made form liquifying rocks/minerals and spinning/blowing them...think cotton candy.
For the purposes of hydroponics it is desirable to have a "relatively" dense product becuse one of the mIn reasons for using it is for structural stability of the plant.
You indicate that you have some insulation leftover...it could work.
If I were going to try it I would compress it "moderately" and use my best judgement on stability and also porousity (I know, not a word, prolly)....water needs to be able to wick and evaporate through it.
I use rockwool cubes that are like 2" x 2" for my clones and they are pretty dense and they fit pretty snugly in my netpots.
Experiment, man..that is what it is all about!