Grizzly, you can use luke warm water to clean them. Feel the water with your hand, if it feels hot, it's too hot. Small temperature changes don't cause a lot of stress on the glass, but big ones do. Think of it this way, you have a room temp carboy, and then you start filling it with hot water. The bottom starts to heat up, but the sides are still at room temperature. The glass where the temp begins to rise will expand, causing stress in the area that boarders the hot and cold glass. As you fill, the weight of the water is pushing on those stressed areas. Then, if you try moving it, you add more stress by shifting the weight around. Any of those actions are enough to break a carboy. You're lucky that the carboy filled with hot water didn't break when it was hit by the dropped one, but maybe the entire carboy was at the same temp, relieving the temperature stresses. Be vary careful using it in the future, there could be cracks that you can't see where it might fail.
As far as cleaning, you can still use PBW or Oxy-Clean, but as I said, use luke warm water. I fill mine, let it soak overnight, and then use a carboy brush. It comes clean without a lot of scrubbing.
Also, read through my thread about
my broken carboy. There is an idea in there that I'm using on my carboys that could possibly save someone some severe injuries. I haven't broken another one yet, but I'm hoping that when I do, that stuff saves me another trip to the ER. (Disclaimer: I have tested the product on a carboy, and it held pretty well. But your mileage may vary, so use at your own risk.)