Will your fish burn it-self, IMO
yes it will. If a pleco or synodontis lays on a heater and the heater comes on they won't move, and in the end it will burn into there skin.
Put a heater into a bucket of water and turn it on full and see how long you can hold on to it.
If your fish are laying on it for long periods eventually it will burn them, i would put a heater guard in the tank.
When it comes to water changes, if your water level goes on to the heater so that the heater eliment is above the water then it will blow. Check to see how far down you water is when you do your change.
If it is close to the eliment it is better to be safe that sorry, if it is nowhere near then you have no worrys.