Sun Pass will work on EZ Pass in FL, however I can't say if the vise-versa will work as I only use Sun Pass. I have traveled all over the state with Sun Pass and never had a problem.
As a note, when you go through the toll, a sign should light up and say "Thank You". (And sometimes a low balance warning, depending on your balance)
Worst case is they will mail you a ticket. I have 'heard' that it can take months to receive it. I cannot verify that from personal experience though...
I agree with Ivylog, just put it on the bottom of your windshield and it will 'see' it as sticking it on the top of windshield will allow it to fall down, just a matter of time. The little suction cups are not great...
I stick mine on glass with dashboard for support, in the lower left near the VIN.
Truthfully, I have had mine fall off glass and lay on floorboard of my car, and it still picked it up...