Coolant must be checked with engine off ALWAYS, and don't do it when it's hot, you can get some serious injuries.
When refilling coolant, you have to do it with engine OFF, at ambient temperature, and you must set your AC to the highest temp possible (hot) and do it BOTH SIDES (driver and passenger side).
Refill it slowly, with the bleeder unscrew a couple of turns to let the air bubbles go outside. When it gets to the maximum recommended, then put the cap on and close the bleeder. Then turn the engine on, and let it run until the temp needle gauge gets to the center or go drive it a few blocks, then let the temperature drop to a safe temp, and re-open the cap and refill it to maximum if necessary.
If you do it correctly you shouldn't need to refill it ever again, unless you have a leak somewhere, or if you are doing some maintenance (every 2 years for coolant).
Use a good quality and organic coolant with the correct mixture of distilled water for your local weather. Don't be cheap on this because you might damage a lot of parts for doing so and it will be more expensive at the end.
If you're asking yourself why you have to set the AC to max temp, like I did when I was told the first time, the reason is because hot temperature is controlled by a couple of electric valves or solenoids that when opened allows hot coolant to run within the system to raise the temperature of the cabin, or when closed no hot water can warm the air of the cabin, so you get cold air only. Those passages need to be filled with coolant as well to work properly.
A good coolant won't let the internal parts get rusted, so despite the amount of liquids running in it, there won't be any damage, at least those caused by rust.
I hope it clarifies your concerns. I had to learn this the bad way.
