Should the cement floor adheres to the concrete slab?
I can't recommend adding a plastic moisture barrier between the pads.  Really if you have moisture in the soil then it needs to work its way out.  Moisture comes out of concrete all the time - we only see the worst case scenarios.

If you leave the plastic in there then the water will get trapped on the bottom slab.  Very good for the short-term.  Long-term though this will cause the bottom slab to decay and break down.  And since you are building on this, that's not a good thing.

I usually advise in drilling down into existing concrete and installing rebar or a mesh unit to join the floors.  To be honest with you I have never installed concrete on top of a floor like the one in your picture.  Common sense would tell me there wouldn't be much movement... but if I was giving out a 10 year warranty with my work I would rebar.