You can isolate it, but if the rest of the floor is not isolated, that may just be more cost. usually the isolation helps in floor absorption when on a slab, since the bass waves can't just pass through to the level below. But again, no harm. My riser is not isolated and it is on the slab. The rest of the floor is just a big area rug with a thick pad. Not ideal, but I didn't have much choice.
I have two layers on mine, but of plywood, which I would recommend over MDF. The edges of MDF chip under heavy use and even carpeted, they could break down over time.
Mine is screwed down so that I can get under there to run wires, if needed. That's come up twice. If it's more permanent, I would glue and screw, but be forewarned, you may have to destroy it if you need in there.
