Safe temperature for desoldering SMD components w/ hot-air rework gun?
Very good idea to practice, practice, and practice some more on salvaged boards until you get competent with new tools. You didn't notice the solder had reached the molten state because there's so little of it used to solder components.

The typical lead-free solder has a melting point around 217 degrees C, so you'll have to get the leads and pads up to that temperature before trying to remove the component. The reason why you need a much higher temperature is because you want to get the solder joints to the melting point as fast as possible. If the hot-air gun is set to a much lower temperature, it'll take a longer time to reach the melting point. The longer the time to raise the temperature of the leads/pads will increase the overall temperature of the component, possibly past the point of destruction. So the technique is to heat it up fast, remove the part and hot-air gun, then put the part where it can cool off. 

Now, if you're removing a part that's already fried due to some other reason, no worries then. Just don't damage the board by overheating the pads and causing them to lift off. If that happens, your headaches are just beginning on this repair.